feat: customizable session driver

[SychO9]

Part of the security roadmap

Changes proposed in this pull request:

• Makes the session driver interchangeable (currently, a driver can be selected by setting session.driver in config.php, in the future probably through an advanced admin page UI).
• Introduces the needed extension API.
• Adds tests.
• Adds session driver to info command.

Reviewers should focus on:

• Currently if the driver is configured to a driver that doesn't exist an exception is thrown, would we rather silently fallback to the default? I feel like that might be more convenient as an admin might disable the extension that provides the driver and crashing with an exception wouldn't be nice.

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.

[luceos]

• Currently if the driver is configured to a driver that doesn't exist an exception is thrown, would we rather silently fallback to the default? I feel like that might be more convenient as an admin might disable the extension that provides the driver and crashing with an exception wouldn't be nice.

I think that makes the most sense. It would however be necessary to add an entry to the logs (error/critical) that provides feedback about this failure to the webmaster.

[askvortsov1]

Looks good to me!