[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/ByteBuffer.swift

@@ -20,7 +20,7 @@ import Foundation
 /// it allows users to write and read data directly from memory thus the use of its
 /// functions should be used
 @frozen
-public struct ByteBuffer {
+public struct ByteBuffer: @unchecked Sendable {
 
   /// Storage is a container that would hold the memory pointer to solve the issue of
   /// deallocating the memory that was held by (memory: UnsafeMutableRawPointer)
@@ -30,7 +30,7 @@ public struct ByteBuffer {
     enum Blob {
       #if !os(WASI)
       case data(Data)
-      case bytes(ContiguousBytes)
+      case bytes(any ContiguousBytes)
       #endif
 
       case byteBuffer(_InternalByteBuffer)