[Feature] Custom resource controller actions

[lindyhopchris]

Adds support for registering custom actions for resources. Although this deviates from the JSON:API spec, it is something that is regularly request or queried via Github issues.

Example:

$server->resource('posts')->actions('-actions', function ($actions) {
    $actions->delete('purge');
    $actions->withId()->post('publish');
});

Registers the following routes:

DELETE /api/v1/posts/-actions/purge
    -> PostController@purge
POST /api/v1/posts/{post}/-actions/publish
    -> PostController@publish

Supports all the usual methods - get, post, patch, put, delete and options.

It is recommended to use a prefix that avoids potential collisions with any future additions to the JSON:API spec - so in the example, we use /-actions instead of /actions. The prefix is optional, for example:

$server->resource('posts')->actions(function ($actions) {
    $actions->delete('purge');
    $actions->withId()->post('publish');
});

Registers the following routes:

DELETE /api/v1/posts/purge
POST /api/v1/posts/{post}/publish

The controller method name is camelised, but this can be overidden using the second method argument:

// PostController@fooBar
$actions->delete('foo-bar');

// PostController@foobar
$actions->delete('foo-bar', 'foobar');