[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/FlatBufferObject.swift

@@ -20,6 +20,8 @@ import Foundation
 /// since now we will be serializing native structs into the buffer.
 public protocol NativeStruct {}
 
+public protocol FlatBufferVerifiableNativeStruct: NativeStruct, Verifiable {}
+
 /// FlatbuffersInitializable is a protocol that allows any object to be
 /// Initialized from a ByteBuffer
 public protocol FlatbuffersInitializable {
@@ -35,6 +37,8 @@ public protocol FlatBufferTable: FlatbuffersInitializable,
   var __buffer: ByteBuffer! { get }
 }
 
+public protocol FlatBufferVerifiableTable: FlatBufferTable, Verifiable {}
+
 /// FlatbufferStruct structures all the Flatbuffers structs
 public protocol FlatBufferStruct: FlatbuffersInitializable,
   FlatbuffersVectorInitializable