[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/FlexBuffers/ByteBuffer.swift

@@ -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)

swift/Sources/FlexBuffers/FlexBufferType.swift

@@ -16,7 +16,7 @@
 
 import Foundation
 
-public enum FlexBufferType: UInt64 {
+public enum FlexBufferType: UInt64, Sendable {
   case null = 0
   /// Variable width signed integer: `Int8, Int16, Int32, Int64`
   case int = 1

swift/Sources/FlexBuffers/Utils/BitWidth.swift

@@ -17,7 +17,7 @@
 import Foundation
 
 @usableFromInline
-enum BitWidth: UInt64, CaseIterable {
+enum BitWidth: UInt64, CaseIterable, Sendable {
   case w8 = 0
   case w16 = 1
   case w32 = 2

swift/Sources/FlexBuffers/Utils/Value.swift

@@ -20,10 +20,10 @@ import Foundation
 import Common
 #endif
 
-public struct Value: Equatable {
+public struct Value: Equatable, Sendable {
 
   @usableFromInline
-  enum Union: Equatable {
+  enum Union: Equatable, Sendable {
     case i(Int64)
     case u(UInt64)
     case f(Double)

swift/Sources/FlexBuffers/Writer/FlexBuffersWriter.swift

@@ -977,6 +977,9 @@ fileprivate struct Stack: RandomAccessCollection {
   mutating func removeAll(keepingCapacity keepCapacity: Bool = false) {
     count = 0
     if !keepCapacity {
+      let ptr = storage.memory
+      defer { ptr.deallocate() }
+
       capacity = Self.initialCapacity
       storage.memory = UnsafeMutableRawPointer.allocate(
         byteCount: capacity,