Target SiriIntents: Split IntentHandler into smaller files (#6203)

* Add protocol `ContactResolving`

* Let the `IntentHandler` implement `ContactResolving` (#6203)

Nothing has changed about the implementation itself; this prepares
the separation of this logic into a dedicated unit.

* Prepare the separation of the contact resolver from the intents handler (#6203)

* Move the implementation of `ContactResolving` to a dedicated class (#6203)

* Move `ContactResolver` to a dedicated file (#6203)

* Prepare the separation of the `StartAudioCallIntentHandler` from `IntentsHandler` (#6203)

* Move the implementation of `INStartAudioCallIntentHandling` to a dedicated class (#6203)

* Prepare the separation of the `StartVideoCallIntentHandler` from `IntentsHandler` (#6203)

* Move the implementation of `INStartVideoCallIntentHandling` to a dedicated class (#6203)

* Prepare the separation of the `SendMessageIntentHandler` from `IntentsHandler` (#6203)

* Move the implementation of `INSendMessageIntentHandling` to a dedicated class (#6203)

* Remove unused property (#6203)

* Return `nil` if the requested intent cannot be handled (#6203)

* Initialize the intent handlers _after_ everything else is configured (#6203)

In `init()`, there might be some configuration being done that is
required for the handlers.

* Add changelog entry

* Move curly braces in Objective-C to dedicated lines

This ensures that the code follows the style that is present in other Objective-C files.

Co-authored-by: Doug <6060466+pixlwave@users.noreply.github.com>

* Inject the `ContactResolver` into the intent handlers during initialization

In #6365, @pixlwave pointed out that

> If the resolver ever gained a cache or stored properties it would
> help keep the memory usage down in the extension

* Prefer forward-declaration over import in Objective-C header files

Per @pixlwave, this helps prevent build failures:

> We had random cycle errors while building a while back and it was
> eventually solved by removing all imports of `GeneratedInterface-Swift.h`
> in every [Objective-C header] file.

Co-authored-by: Doug <6060466+pixlwave@users.noreply.github.com>

SiriIntents/IntentHandlers/SendMessage/SendMessageIntentHandler.h

@@ -0,0 +1,29 @@
+// 
+// Copyright 2022 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 <Foundation/Foundation.h>
+@import Intents;
+@protocol ContactResolving;
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface SendMessageIntentHandler : NSObject <INSendMessageIntentHandling>
+
+- (instancetype)initWithContactResolver:(id<ContactResolving>)contactResolver;
+
+@end
+
+NS_ASSUME_NONNULL_END