Support multiple databases for install generator

[fractaledmind]

Currently, there is no way to specify that the migration files be installed into a separate database from the default. This is problematic especially for SQLite apps, since the recommended approach is to use a separate SQLite database for the queue.

This change supports the existing behavior as well as specifying the --database option or the --db option.

[fractaledmind]

@rosa: I can't figure out how the solid_queue:install:migrations Rake task is defined, so I can't then figure out if there is any way to specify a specific database for that task or not.

[fractaledmind]

I will also add to the README and some tests

[rosa]

Thank you so much @fractaledmind! This is related to #144.

The solid_queue:install:migrations task is given by Rails's Engine and yes! It does allow specifying a database via the DATABASE environment variable, it was added in rails/rails#48579.

Thank you again! 🙏 🙇‍♀️