Improve the installer to account for an existing multi-existing db configuration

[dhh]

With 1.0.3, we're modifying the config/database.yml file automatically. That works well for new out-of-the-box Rails apps. But if you've already modified that file with a multi-DB setup, or otherwise, we'll be overwriting that. This needs to be fixed before Rails 8, as both Solid Cache and Solid Queue will need to be configured out of the box.

[dhh]

If you wanted to take a stab at this @fractaledmind, here's the issue to track it.

[dhh]

Solved via https://github.com/rails/solid_cache/commit/0a36decd3e59c3cfab6efe67e42ba8bc910a7ed2 and https://github.com/rails/rails/commit/cae25f3c2e5c7d09f90fceefd448ad8d819028f1