Remove curl dependency and disable OAuth Bearer SASL mechanism

# Motivation
The curl dependency is just necessary for the OAuth Bearer SASL mechanism. Let's remove that configuration option for now.

# Modification
Removes the curl dependency.

Author: FranzBusch

Diff for: Package.swift

@@ -72,7 +72,6 @@ let package = Package(
                 .define("_GNU_SOURCE", to: "1"), // Fix build error for Swift 5.9 onwards
             ],
             linkerSettings: [
-                .linkedLibrary("curl"),
                 .linkedLibrary("sasl2"),
                 .linkedLibrary("z"), // zlib
             ]

Diff for: Sources/Crdkafka/custom/config/config.h

@@ -21,7 +21,7 @@
 #define ENABLE_ZSTD 1
 #define ENABLE_SSL 1
 #define ENABLE_GSSAPI 1
-#define ENABLE_CURL 1
+#define ENABLE_CURL 0
 #define ENABLE_DEVEL 0
 #define ENABLE_VALGRIND 0
 #define ENABLE_REFCNT_DEBUG 0

Diff for: Sources/Kafka/Configuration/KafkaConfiguration+Security.swift

@@ -309,7 +309,7 @@ extension KafkaConfiguration {
             }
         }
 
-        public struct OAuthBearerMethod: Sendable, Hashable {
+        struct OAuthBearerMethod: Sendable, Hashable {
             internal enum _OAuthBearerMethod: Sendable, Hashable {
                 case `default`(
                     configuration: String?
@@ -337,7 +337,7 @@ extension KafkaConfiguration {
             ///     For example: `principalClaimName=azp principal=admin scopeClaimName=roles scope=role1,role2 lifeSeconds=600`.
             ///     In addition, SASL extensions can be communicated to the broker via `extension_NAME=value`.
             ///     For example: `principal=admin extension_traceId=123`
-            public static func `default`(configuration: String? = nil) -> OAuthBearerMethod {
+            static func `default`(configuration: String? = nil) -> OAuthBearerMethod {
                 return OAuthBearerMethod(_internal: .default(configuration: configuration))
             }
 
@@ -359,7 +359,7 @@ extension KafkaConfiguration {
             ///     - scope: The client uses this to specify the scope of the access request to the broker.
             ///     - extensions: Allow additional information to be provided to the broker.
             ///     Comma-separated list of key=value pairs. E.g., "supportFeatureX=true,organizationId=sales-emea".
-            public static func oidc(
+            static func oidc(
                 configuration: String? = nil,
                 clientID: String,
                 clientSecret: String,
@@ -419,7 +419,8 @@ extension KafkaConfiguration {
         }
 
         /// Use the OAUTHBEARER mechanism.
-        public static func oAuthBearer(method: OAuthBearerMethod) -> SASLMechanism {
+        // This is currently disabled since it requires a curl dependency otherwise.
+        static func oAuthBearer(method: OAuthBearerMethod) -> SASLMechanism {
             return SASLMechanism(
                 _internal: .oAuthBearer(method: method)
             )