feat: Support for user-costomizable user agent strings.

Doable by setting the `gitoxide.userAgent` variable.

Author: Byron

Diff for: git-repository/src/config/cache/access.rs

@@ -44,6 +44,20 @@ impl Cache {
             .copied()
     }
 
+    /// Returns a user agent for use with servers.
+    pub(crate) fn user_agent_tuple(&self) -> (&'static str, Option<Cow<'static, str>>) {
+        let agent = self
+            .user_agent
+            .get_or_init(|| {
+                self.resolved
+                    .string("gitoxide", None, "userAgent")
+                    .map(|s| s.to_string())
+                    .unwrap_or_else(|| crate::env::agent().into())
+            })
+            .to_owned();
+        ("agent", Some(git_protocol::fetch::agent(agent).into()))
+    }
+
     pub(crate) fn personas(&self) -> &identity::Personas {
         self.personas
             .get_or_init(|| identity::Personas::from_config_and_env(&self.resolved, self.git_prefix))

Diff for: git-repository/src/config/cache/init.rs

@@ -136,6 +136,7 @@ impl Cache {
             xdg_config_home_env,
             home_env,
             lenient_config,
+            user_agent: Default::default(),
             personas: Default::default(),
             url_rewrite: Default::default(),
             #[cfg(any(feature = "blocking-network-client", feature = "async-network-client"))]
@@ -177,6 +178,7 @@ impl Cache {
         self.object_kind_hint = object_kind_hint;
         self.reflog = reflog;
 
+        self.user_agent = Default::default();
         self.personas = Default::default();
         self.url_rewrite = Default::default();
         self.diff_algorithm = Default::default();

Diff for: git-repository/src/config/mod.rs

@@ -124,6 +124,8 @@ pub(crate) struct Cache {
     pub use_multi_pack_index: bool,
     /// The representation of `core.logallrefupdates`, or `None` if the variable wasn't set.
     pub reflog: Option<git_ref::store::WriteReflog>,
+    /// The configured user agent for presentation to servers.
+    pub(crate) user_agent: OnceCell<String>,
     /// identities for later use, lazy initialization.
     pub(crate) personas: OnceCell<identity::Personas>,
     /// A lazily loaded rewrite list for remote urls

Diff for: git-repository/src/remote/connection/fetch/receive_pack.rs

@@ -51,22 +51,30 @@ where
     /// We explicitly don't special case those refs and expect the user to take control. Note that by its nature,
     /// force only applies to refs pointing to commits and if they don't, they will be updated either way in our
     /// implementation as well.
+    ///
+    /// ### Configuration
+    ///
+    /// - `gitoxide.userAgent` is read to obtain the application user agent for git servers and for HTTP servers as well.
     pub fn receive(mut self, should_interrupt: &AtomicBool) -> Result<Outcome, Error> {
         let mut con = self.con.take().expect("receive() can only be called once");
 
         let handshake = &self.ref_map.handshake;
         let protocol_version = handshake.server_protocol_version;
 
         let fetch = git_protocol::fetch::Command::Fetch;
-        let fetch_features = fetch.default_features(protocol_version, &handshake.capabilities);
+        let progress = &mut con.progress;
+        let repo = con.remote.repo;
+        let fetch_features = {
+            let mut f = fetch.default_features(protocol_version, &handshake.capabilities);
+            f.push(repo.config.user_agent_tuple());
+            f
+        };
 
         git_protocol::fetch::Response::check_required_features(protocol_version, &fetch_features)?;
         let sideband_all = fetch_features.iter().any(|(n, _)| *n == "sideband-all");
         let mut arguments = git_protocol::fetch::Arguments::new(protocol_version, fetch_features);
         let mut previous_response = None::<git_protocol::fetch::Response>;
         let mut round = 1;
-        let progress = &mut con.progress;
-        let repo = con.remote.repo;
 
         if self.ref_map.object_hash != repo.object_hash() {
             return Err(Error::IncompatibleObjectHash {

Diff for: git-repository/src/remote/connection/ref_map.rs

@@ -65,6 +65,10 @@ where
     ///
     /// Due to management of the transport, it's cleanest to only use it for a single interaction. Thus it's consumed along with
     /// the connection.
+    ///
+    /// ### Configuration
+    ///
+    /// - `gitoxide.userAgent` is read to obtain the application user agent for git servers and for HTTP servers as well.
     #[allow(clippy::result_large_err)]
     #[git_protocol::maybe_async::maybe_async]
     pub async fn ref_map(mut self, options: Options) -> Result<fetch::RefMap, Error> {
@@ -157,11 +161,13 @@ where
             Some(refs) => refs,
             None => {
                 let specs = &self.remote.fetch_specs;
+                let agent_feature = self.remote.repo.config.user_agent_tuple();
                 git_protocol::fetch::refs(
                     &mut self.transport,
                     outcome.server_protocol_version,
                     &outcome.capabilities,
-                    |_capabilities, arguments, _features| {
+                    move |_capabilities, arguments, features| {
+                        features.push(agent_feature);
                         if filter_by_prefix {
                             let mut seen = HashSet::new();
                             for spec in specs {