feat: Add support for pgsql's search_path

[johannsa]

Fixes #4240

Changes proposed in this pull request:
Added support for specifying and using pgsql's search_path

Reviewers should focus on:

• Some changes to existing classes and method signatures.
• Some changes to UI:
  • Addition of a "Schema" field.
  • Triggering the update of the UI upon DOM loading, so that the field is only displayed for the relevant driver.

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
  • Not acceptable: Install in "public" schema, then move tables to other schema and finally change config.php.
• For core PRs, does this need to be in core, or could it be in an extension?
  • Needs to be in core as the issue/limitation exists since install.
• 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).
  • There is no such command on my composer.jsons
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)

[johannsa]

I am working on those failed test ✅ Done!