[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/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,