[Swift] Flexbuffers native swift port (#8577)

* Offical Swift port for FlexBuffers

This is the offical port for FlexBuffers within
swift, and it introcudes a Common Module where code
is shared between flatbuffers and flexbuffers.

Writing most supported values like maps, vectors,
nil and scalars into a flexbuffer buffer. And includes
tests to verify that its similar to cpp

* Reading a flexbuffer

Implementing reading from a flexbuffer, enabling
most of the buffers features, like most types, maps, vectors,
typedvectors, and fixedtypedvectors.

Currently, if an offset/object cant be read we default to a swift
nil instead of the default flexbuffers 'null' with all values.

* Fixes bazel breaking due to new project structure

Address warnings within the library

* Adds comment on why we added the code & properly enforce the amout of bytes needed

swift/Sources/FlexBuffers/Reader/FlexBufferVector.swift

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2024 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import Foundation
+
+protocol FlexBufferVector: Sized & FlexBufferContiguousBytes {
+  subscript(index: Int) -> Reference? { get }
+}
+
+extension FlexBufferVector {
+  public func jsonBuilder(json: inout String) {
+    json += "["
+    for i in 0..<count {
+      if let val = self[i]?.jsonString() {
+        let comma = i == count - 1 ? "" : ", "
+        json += "\(val)\(comma)"
+      }
+    }
+    json += "]"
+  }
+}
+
+public protocol FlexBufferContiguousBytes {
+  var byteBuffer: ByteBuffer { get }
+  var offset: Int { get }
+  var count: Int { get }
+
+  func withUnsafeRawBufferPointer<Result>(
+    _ body: (UnsafeRawBufferPointer) throws -> Result) rethrows -> Result
+}
+
+extension FlexBufferContiguousBytes {
+  public func withUnsafeRawBufferPointer<Result>(
+    _ body: (UnsafeRawBufferPointer) throws -> Result) rethrows -> Result
+  {
+    try byteBuffer.withUnsafePointerToSlice(
+      index: offset,
+      count: count,
+      body: body)
+  }
+}