Refactor config screens' internal code

Problem/Motivation

Drupal 10.2 (or so, I don't remember) has had extensions of / automatically added functionality in config forms.

Basically: everything related to ConfigFormBase::CONFIG_KEY_TO_FORM_ELEMENT_MAP.

The (currently one?) thing this is useful for: if config values are overridden in code / settings.php, this fact will be shown in the form.

Proposed resolution

Refactor to use CONFIG_KEY_TO_FORM_ELEMENT_MAP etc. (If I remember correctly, the 'save' will be automatic so save() can be removed, but there will need to be a lot of code that needs to be moved into a config mapping/array containing closures.)

Then test if all Pronovix/ddev-saml-idp's overrides are shown correctly.

This is not a very easy task, given there are now two forms (split up because the screen grew so large) for editing the same config object, and there are many overrides of a 'regular simple' editing experience. For instance, the keys. I'm not 100% sure if this will work cleanly.