[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/Message.swift

@@ -16,50 +16,52 @@
 
 import Foundation
 
-/// FlatBufferGRPCMessage protocol that should allow us to invoke
-/// initializers directly from the GRPC generated code
-public protocol FlatBufferGRPCMessage {
-  /// Size of readable bytes in the buffer
-  var size: Int { get }
-
-  init(byteBuffer: ByteBuffer)
-
-  @discardableResult
-  @inline(__always)
-  func withUnsafeReadableBytes<T>(
-    _ body: (UnsafeRawBufferPointer) throws
-      -> T) rethrows -> T
+enum FlatbuffersGRPCError: Error {
+  case finishedNotCalledOnBuilder
 }
 
-/// Message is a wrapper around Buffers to to able to send Flatbuffers `Buffers` through the
-/// GRPC library
-public struct Message<T: FlatBufferTable>: FlatBufferGRPCMessage {
-  internal var buffer: ByteBuffer
+public protocol GRPCVerifiableMessage<Message> {
+  associatedtype Message
 
-  /// Returns the an object of type T that would be  read from the buffer
-  public var object: T {
-    T.init(
-      buffer,
-      o: Int32(buffer.read(def: UOffset.self, position: buffer.reader)) &+
-        Int32(buffer.reader))
-  }
+  init(pointer: UnsafeRawBufferPointer)
+  init(byteBuffer: ByteBuffer)
+
+  func decode() throws -> Message
+  func withUnsafeReadableBytes<Data>(
+    _ body: (UnsafeRawBufferPointer) throws
+      -> Data) rethrows -> Data
+}
+
+public struct GRPCMessage<
+  Table: FlatBufferVerifiableTable
+>: Sendable, GRPCVerifiableMessage {
+  public typealias Message = Table
+
+  private let buffer: ByteBuffer
 
   public var size: Int { Int(buffer.size) }
 
-  /// Initializes the message with the type Flatbuffer.Bytebuffer that is transmitted over
-  /// GRPC
-  /// - Parameter byteBuffer: Flatbuffer ByteBuffer object
+  public init(pointer: UnsafeRawBufferPointer) {
+    buffer = ByteBuffer(
+      copyingMemoryBound: pointer.baseAddress!,
+      capacity: pointer.count)
+  }
+
+  public init(builder: inout FlatBufferBuilder) throws {
+    guard builder.finished else {
+      throw FlatbuffersGRPCError.finishedNotCalledOnBuilder
+    }
+
+    buffer = builder.sizedBuffer
+  }
+
   public init(byteBuffer: ByteBuffer) {
     buffer = byteBuffer
   }
 
-  /// Initializes the message by copying the buffer to the message to be sent.
-  /// from the builder
-  /// - Parameter builder: FlatbufferBuilder that has the bytes created in
-  /// - Note: Use  `builder.finish(offset)` before passing the builder without prefixing anything to it
-  public init(builder: inout FlatBufferBuilder) {
-    buffer = builder.sizedBuffer
-    builder.clear()
+  public func decode() throws -> Table {
+    var buf = buffer
+    return try getCheckedRoot(byteBuffer: &buf)
   }
 
   @discardableResult
@@ -73,3 +75,30 @@ public struct Message<T: FlatBufferTable>: FlatBufferGRPCMessage {
     }
   }
 }
+
+@available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
+public struct FlatBuffersMessageSerializer<
+  Message: GRPCVerifiableMessage
+>: Sendable {
+  public init() {}
+
+  public func serialize<Data>(
+    message: Message,
+    _ completion: (UnsafeRawBufferPointer) throws -> Data) throws -> Data
+  {
+    return try message.withUnsafeReadableBytes {
+      try completion($0)
+    }
+  }
+}
+
+@available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
+public struct FlatBuffersMessageDeserializer<
+  Message: GRPCVerifiableMessage
+>: Sendable {
+  public init() {}
+
+  public func deserialize(pointer: UnsafeRawBufferPointer) throws -> Message {
+    Message.init(pointer: pointer)
+  }
+}