Reduce duplication of DTO code

[bartekpacia]

TL;DR Classes representing "commands", such as TapCommand, HandlePermissionCommand, etc. are written 3 times: in Dart (not exactly, though), Kotlin, and Swift. We should fix that.

Let's take a closer look at a very simple grantPermissionWhenInUse method. That's how it's implemented on the Dart side, in patrol:

Future<void> grantPermissionWhenInUse() async {
  await _wrapPost(
    'handlePermission',
    <String, String>{'code': 'WHILE_USING'},
  );
}

And in Kotlin:

@Serializable
data class PermissionCommand(var code: String)

And in Swift:

struct PermissionCommand: Codable {
  var code: String
}

You can easily see how this can quickly become unmanageable when more Commands are added.

I've seen that other new interesting mobile testing tools (Maestro by mobile.dev, convenient_test) use Protocol Buffers. I'm wondering if we could also use them to generate code in Dart, Swift, and Kotlin from protobuf.

cc @jakubfijalkowski

[bartekpacia]

Done in #338!