Leverage configuration to specify targeted schemas

[foxted]

Is your feature request related to a problem? Please describe.
When working with other schemas not impacted by Supabase migrations (e.g. some other schemas from another app, with its own migration system, like PayloadCMS), it becomes tedious to have to specify the --schema flag for the supabase db diff and other commands. Particularly when working on a project with other folks, as there is a risk of them running supabase db diff and including migrations that shouldn't be there.

Describe the solution you'd like
Allow a way to specify via configuration which schemas should be impacted by these commands (instead of the flag). This way, this configuration would be part of version control and supabase db diff and other commands would use it by default, preventing unwanted migrations being generated, or in the case of supabase db reset, preventing schemas to be reset when they shouldn't be (dependant on #2626).

Describe alternatives you've considered
Use the README to communicate that folks should only use the supabase:diff npm script to generate migration, as this script contains the --schema flag.