[Swift] Migrate to swift 6.0 and Implements support gRPC v2 (#8983)

* Migrate to swift 6.0 & swift-gRPC 2.0

The following migrates to swift 6.0, and also
migrate to swift-grpc 2.0 that uses swift-nio
under the hood to provide nicer API and async await

Adds sendable to enum & update @_implementationOnly imports to use internal imports

* Address PR comments regarding misspelling & proper method naming.

grpc/examples/greeter_v2.fbs

@@ -0,0 +1,16 @@
+namespace models;
+
+table HelloResponse {
+  message:string;
+}
+
+table HelloRequest {
+  name:string;
+}
+
+rpc_service Greeter {
+  Get(HelloRequest):HelloResponse;
+  Collect(HelloRequest):HelloResponse (streaming: "client");
+  Expand(HelloRequest):HelloResponse (streaming: "server");
+  Update(HelloRequest):HelloResponse (streaming: "bidi");
+}