[Swift] Implements FlatbuffersVector which confirms to RandomAccessCollection (#8752)

* Implements FlatbuffersVector in swift

Implements FlatbuffersVector which confirms to RandomAccessCollection,
this would give us semi-native sugary syntax to all the arrays in swift port.

This work will also be the foundation of using arrays in swift

* Fix failing tests for Swift

swift/Sources/FlatBuffers/Vectors/UnionFlatbufferVector.swift

@@ -0,0 +1,62 @@
+/*
+ * 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
+
+public struct UnionFlatbufferVector {
+
+  private let bb: ByteBuffer
+  private let byteSize: Int
+  private let startPosition: Int32
+  public let count: Int
+
+  init(
+    bb: ByteBuffer,
+    startPosition: Int32,
+    count: Int,
+    byteSize: Int)
+  {
+    self.bb = bb
+    self.byteSize = byteSize
+    self.count = count
+    self.startPosition = startPosition
+  }
+
+  public var startIndex: Int {
+    0
+  }
+
+  public var endIndex: Int {
+    count
+  }
+
+  public var isEmpty: Bool {
+    count == 0
+  }
+
+  public subscript(
+    position: Int,
+    Type: FlatbuffersVectorInitializable
+      .Type) -> FlatbuffersVectorInitializable
+  {
+    guard position < count else {
+      fatalError(
+        "Trying to read element at index \(position) in a vector of size \(count)")
+    }
+    let index = startPosition &+ Int32(position &* byteSize)
+    return Type.readFrom(byteBuffer: bb, index: Int(index))
+  }
+}