[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.

swift/Sources/FlatBuffers/TableVerifier.swift

@@ -115,7 +115,7 @@ public struct TableVerifier {
     unionKeyName: String,
     fieldName: String,
     required: Bool,
-    completion: @escaping (inout Verifier, T, Int) throws -> Void) throws
+    completion: (inout Verifier, T, Int) throws -> Void) throws
     where T: UnionEnum
   {
     let keyPos = try dereference(key)