[2.x] feat: DatabaseQueue support, queue settings in advanced page

[imorland]

Fixes #0000

Changes proposed in this pull request:

Docs: flarum/docs#495

By default, Flarum continues to use the synchronous queue. No action needed.

// config.php - Default behavior (no queue config)

<?php return [
    'url' => 'https://forum.example.com',
    'database' => [...],
    // Jobs execute synchronously (same as before)
];

For forum admins who wish to use the DatabaseQueue, this can be enabled:

// config.php - Specify database as the queue driver

<?php return [
    'url' => 'https://forum.example.com',
    'database' => [...],
    
    'queue' => [
        'driver' => 'database',
    ],
];

Also allows for 3rd party extensions to provide/bind their own queue implementation as required

Reviewers should focus on:

Screenshot
With DatabaseQueue active:

With default SyncQueue active:

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)