Create type-safe abstraction for declarative settings

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Is your feature request related to a problem? Please describe.
Settings cause trouble in many places as it includes a lot of layers.
You have to make sure to read and write the correct keys, use the correct types and build your own settings UI.

Describe the solution you'd like
A PHP abstraction for declarative settings schemas.
We have declarative settings that can be used to manage settings in type-safe and declarative way. So far only a schema based implementation is available which is tedious to use in PHP as no simple type-safety using classes, methods, parameters and so on exists.
Bugs like https://github.com/nextcloud/server/pull/47674 can be ruled out entirely by relying on declarative settings.
It reduces the load on both frontend and backend engineers as it simplifies the process of using settings and removes an entire part of frontend work.

The issue does not name any files, tests, or entry points. Start by locating the existing schema-based declarative settings implementation and reviewing how PHP settings are currently read, written, typed, and exposed to the frontend. Done means a defined type-safe PHP abstraction with a clear schema and integration scope.