Add support for non-streaming APIs on macOS 11 (google-gemini#84)

Package.swift

@@ -21,7 +21,7 @@ let package = Package(
   name: "generative-ai-swift",
   platforms: [
     .iOS(.v15),
-    .macOS(.v12),
+    .macOS(.v11),
     .macCatalyst(.v15),
   ],
   products: [

Sources/GoogleAI/Chat.swift

@@ -65,6 +65,7 @@ public class Chat {
   }
 
   /// See ``sendMessageStream(_:)-4abs3``.
+  @available(macOS 12.0, *)
   public func sendMessageStream(_ parts: PartsRepresentable...)
     -> AsyncThrowingStream<GenerateContentResponse, Error> {
     return sendMessageStream([ModelContent(parts: parts)])
@@ -74,6 +75,7 @@ public class Chat {
   /// and response will be added to the history. If unsuccessful, history will remain unchanged.
   /// - Parameter content: The new content to send as a single chat message.
   /// - Returns: A stream containing the model's response or an error if an error occurred.
+  @available(macOS 12.0, *)
   public func sendMessageStream(_ content: [ModelContent])
     -> AsyncThrowingStream<GenerateContentResponse, Error> {
     return AsyncThrowingStream { continuation in

Sources/GoogleAI/GenerativeAIService.swift

@@ -51,6 +51,7 @@ struct GenerativeAIService {
     return try parseResponse(T.Response.self, from: data)
   }
 
+  @available(macOS 12.0, *)
   func loadRequestStream<T: GenerativeAIRequest>(request: T)
     -> AsyncThrowingStream<T.Response, Error> {
     return AsyncThrowingStream { continuation in

Sources/GoogleAI/GenerativeModel.swift

@@ -138,6 +138,7 @@ public final class GenerativeModel {
   /// for conforming types).
   /// - Returns: A stream wrapping content generated by the model or a ``GenerateContentError``
   ///     error if an error occurred.
+  @available(macOS 12.0, *)
   public func generateContentStream(_ parts: PartsRepresentable...)
     -> AsyncThrowingStream<GenerateContentResponse, Error> {
     return generateContentStream([ModelContent(parts: parts)])
@@ -148,6 +149,7 @@ public final class GenerativeModel {
   /// - Parameter content: The input(s) given to the model as a prompt.
   /// - Returns: A stream wrapping content generated by the model or a ``GenerateContentError``
   ///     error if an error occurred.
+  @available(macOS 12.0, *)
   public func generateContentStream(_ content: [ModelContent])
     -> AsyncThrowingStream<GenerateContentResponse, Error> {
     let generateContentRequest = GenerateContentRequest(model: modelResourceName,