Message type "plugin" system

[nvahalik]

In order to create new message types, the following files have to be modified:

• Message.php
• MessageTransformer.php
• MessageResource.php

This means forking the project and making changes that almost negate the ability to use the package unless you are 100% OK with the existing message types.

By making the message type relatable to a plugin system, we could make extending the message system a lot simpler while allowing the core to improve without having to fork it. Our internal chat system currently does this and it works well. The solution might look something like this:

• Creation of a Message type resolver
• Move 'type' from const into a repository.
• Exchange use of dedicated endpoints (images/documents) for a generic filter/search controller
• Migrate static functions in the Message Transphormer to individual classes
• Register classes with Message type resolve
• Make relevant updates to permission system

These changes would make creation of new types of messages as easy as:

1. Creating a new class
2. Implementing one or more methods on the class
3. Registering the class with the type resolver

Example

class AutomatedReply implements MessageTypeInterface
{
    public function make(Thread $thread, Provider $provider)
    {
        return ['subject' => 'This is an automated reply.'];
    }

    // public function transform(string $body) - if you want to transform the body itself
    // public function transformResource() - if you want to transform the resource
    // public search() - modify search queries?
}

and then in a provider somewhere:

MessageTypeProvider::registerType('automated_reply', AutomatedReply::class);
// Register again because maybe you want several types of automated replies.
MessageTypeProvider::registerType('custom_one', AutomatedReply::class);

While the initial work would be somewhat involved, I think the long term benefits would be huge.

[nvahalik]

@RTippin Initial POC in #46

Would really appreciate a 👍🏻 or 👎🏻 if this is an approach worth considering.