Fixes #32286 - new settings definition DSL

Introduces a new DSL to define settings, so we don't need to define
STI classes for every setting category and we have more control over
settings from the plugins.

Refs #32286 - setting DSL switch preparation

Removes some unused methods and tests.
This also add collections to the old collection registry to work before we hold them in registry only.

Refs #32286 - load definition even in migrations

Loads definitions on every start and makes sure the definitions are always loaded.
This makes it possible to work with settings in migrations.
Only values needs the database, so this is safe to do without any check.