Added addon request system to request data from an addon in spigot plugins

[HyKurtis]

So I've been trying to directly get a players island level from one of my plugins but I've been having issues because spigot plugins can not use classes from Addons. (This was discussed in this issue: #352)

I see there is an option to listen for AddonEvent to receive data, but this would requiring caching of any data received, and this could be a struggle if wanting to obtain offline player levels (as it was in my case)

I've added this system so that you can request data from Addons, and Addons can add handlers for these requests.

Here is an example of a request handler (to be put in an Addons code), for responding to a level request. This code would be ran through the main class of the Addon using #registerRequestHandler.

Here is an example of request to an Addon (to be put in a spigot plugin), to request a player's island level.

I've tested this myself and it works perfectly. This would be an amazing help for developers trying to obtain data from Addons as it would remove the need to cache data after listening through AddonEvent.

I made an effort to maintain the code style of your plugin, there shouldn't be any inconsistencies.

I hope you consider my pull request.

[tastybento]

Thanks for the submission, we'll look at it!

[Poslovitch]

I approve. This PR is well documented and follows our - implicit - code guidelines.