feat: allow specifying extensions when installing an instance

[SychO9]

Changes proposed in this pull request:
Useful when doing migrations when more than the default extensions are required to migrate the data to flarum. This allows quickly spinning up a flarum database with the necessary schema.

This use case was also raised by the work being done in https://github.com/linc/nitro-porter

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]

What an easy fix/addition 👍