ci: allow custom actions runner to be defined

[DavideIadeluca]

Fixes #0000
Companion of #3988

Changes proposed in this pull request:
Allow custom GitHub Actions runner to be defined in the reusable frontend & backend workflows. This allows third-party vendors to optionally pass a new input runner_type which specifies on which machine the jobs are being run (be it a custom GitHub-hosted runner or a self-hosted one).

Hypothetical example of a calling workflow:

name: ACME Foobar PHP

on: [workflow_dispatch, push, pull_request]

jobs:
  run:
    uses: flarum/framework/.github/workflows/REUSABLE_backend.yml@1.x
    with:
      enable_backend_testing: true
      enable_phpstan: true
      php_versions: '["8.0", "8.1", "8.2", "8.3"]'
      runner_type: self-hosted

      backend_directory: . 

It's important to note that this new input is optional and will default to ubuntu-latest if not defined by the calling workflow.

Reviewers should focus on:

Screenshot

QA

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)
• Related core extension PRs: (Remove if irrelevant)