Add a certificate validator hook. (#3069)

* Add a certificate validator hook.

* General tidy up of AppHooks.

* Don't worry about the generic call links.

ElementX/Sources/AppHooks/Hooks/ClientBuilderHook.swift

@@ -0,0 +1,25 @@
+//
+// Copyright 2024 New Vector Ltd
+//
+// 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 MatrixRustSDK
+
+protocol ClientBuilderHookProtocol {
+    func configure(_ builder: ClientBuilder) -> ClientBuilder
+}
+
+struct DefaultClientBuilderHook: ClientBuilderHookProtocol {
+    func configure(_ builder: ClientBuilder) -> ClientBuilder { builder }
+}