[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/FlatBufferBuilder.swift

@@ -45,7 +45,7 @@ public struct FlatBufferBuilder {
   /// Dictonary that stores a map of all the strings that were written to the buffer
   private var stringOffsetMap: [String: Offset] = [:]
   /// A check to see if finish(::) was ever called to retreive data object
-  private var finished = false
+  private(set) var finished = false
   /// A check to see if the buffer should serialize Default values
   private var serializeDefaults: Bool
 
@@ -488,7 +488,7 @@ public struct FlatBufferBuilder {
   ///
   /// - Parameter bytes: bytes to be written into the buffer
   /// - Returns: ``Offset`` of the vector
-  mutating public func createVector(bytes: ContiguousBytes) -> Offset {
+  mutating public func createVector(bytes: any ContiguousBytes) -> Offset {
     bytes.withUnsafeBytes {
       startVector($0.count, elementSize: MemoryLayout<UInt8>.size)
       _bb.push(bytes: $0)