Add GroupRequest callback

Sources/ElixxirDAppsSDK/Callbacks/GroupRequest.swift

+import Bindings
+import XCTestDynamicOverlay
+
+public struct GroupRequest {
+  public init(handle: @escaping (Group) -> Void) {
+    self.handle = handle
+  }
+
+  public var handle: (Group) -> Void
+}
+
+extension GroupRequest {
+  public static let unimplemented = GroupRequest(
+    handle: XCTUnimplemented("\(Self.self)")
+  )
+}
+
+extension GroupRequest {
+  func makeBindingsGroupRequest() -> BindingsGroupRequestProtocol {
+    class CallbackObject: NSObject, BindingsGroupRequestProtocol {
+      init(_ callback: GroupRequest) {
+        self.callback = callback
+      }
+
+      let callback: GroupRequest
+
+      func callback(_ g: BindingsGroup?) {
+        guard let bindingsGroup = g else {
+          fatalError("BindingsGroupRequest.handle received `nil` group")
+        }
+        callback.handle(.live(bindingsGroup))
+      }
+    }
+
+    return CallbackObject(self)
+  }
+}