Use new structured swift representations (#2128)

Motivation:

The new structured Swift representations are more thoroughly tested than
the existing translators, so reimplement the translators in terms of the
structured Swift.

Modifications:

- Reimplement translators

Result:

Most tests remain exactly the same, however the generated client code
changes slightly to make the naming of generic placeholders and closure
parameters clearer.

Author: glbrntt

Diff for: Sources/GRPCCodeGen/CodeGenerationRequest.swift

@@ -335,3 +335,17 @@ extension Name {
     return self.base.replacingOccurrences(of: ".", with: "_")
   }
 }
+
+extension ServiceDescriptor {
+  var namespacedServicePropertyName: String {
+    let prefix: String
+
+    if self.namespace.normalizedBase.isEmpty {
+      prefix = ""
+    } else {
+      prefix = self.namespace.normalizedBase + "_"
+    }
+
+    return prefix + self.name.normalizedBase
+  }
+}

Diff for: Sources/GRPCCodeGen/Internal/StructuredSwift+Server.swift

@@ -97,13 +97,16 @@ extension ExtensionDescription {
     return ExtensionDescription(
       onType: extensionName,
       declarations: [
-        .function(
-          .registerMethods(
-            accessLevel: accessLevel,
-            serviceNamespace: serviceNamespace,
-            methods: methods,
-            serializer: serializer,
-            deserializer: deserializer
+        .guarded(
+          .grpc,
+          .function(
+            .registerMethods(
+              accessLevel: accessLevel,
+              serviceNamespace: serviceNamespace,
+              methods: methods,
+              serializer: serializer,
+              deserializer: deserializer
+            )
           )
         )
       ]

Diff for: Sources/GRPCCodeGen/Internal/StructuredSwift+ServiceMetadata.swift

@@ -302,3 +302,43 @@ extension EnumDescription {
     return description
   }
 }
+
+extension [CodeBlock] {
+  /// ```
+  /// enum <Service> {
+  ///   ...
+  /// }
+  ///
+  /// extension GRPCCore.ServiceDescriptor {
+  ///   ...
+  /// }
+  /// ```
+  package static func serviceMetadata(
+    accessModifier: AccessModifier? = nil,
+    service: ServiceDescriptor,
+    client: Bool,
+    server: Bool
+  ) -> Self {
+    var blocks: [CodeBlock] = []
+
+    let serviceNamespace: EnumDescription = .serviceNamespace(
+      accessModifier: accessModifier,
+      name: service.namespacedGeneratedName,
+      serviceDescriptorProperty: service.namespacedServicePropertyName,
+      client: client,
+      server: server,
+      methods: service.methods
+    )
+    blocks.append(CodeBlock(item: .declaration(.enum(serviceNamespace))))
+
+    let descriptorExtension: ExtensionDescription = .serviceDescriptor(
+      accessModifier: accessModifier,
+      propertyName: service.namespacedServicePropertyName,
+      literalNamespace: service.namespace.base,
+      literalService: service.name.base
+    )
+    blocks.append(CodeBlock(item: .declaration(.extension(descriptorExtension))))
+
+    return blocks
+  }
+}

Diff for: Sources/GRPCCodeGen/Internal/StructuredSwiftRepresentation.swift

@@ -100,7 +100,7 @@ struct ImportDescription: Equatable, Codable, Sendable {
 /// A description of an access modifier.
 ///
 /// For example: `public`.
-internal enum AccessModifier: String, Sendable, Equatable, Codable, CaseIterable {
+package enum AccessModifier: String, Sendable, Equatable, Codable, CaseIterable {
   /// A declaration accessible outside of the module.
   case `public`