nixos/krb5: Cleanup, fix and RFC42-ify

This replaces the krb5 module's options with RFC 42-style krb5.settings
option*, while greatly simplifying the code and fixing a few bugs,
namely:

- NixOS#243068 krb5: Configuration silently gets ignored when set by
  multiple modules
- not being able to use mkIf etc. inside subattributes of
  krb5.libdefaults, e.g. krb5.libdefaults.default_realm = mkIf ...

* leaving an escape hatch in form of krb5.extraConfig in case extra
  syntax is needed (e.g. `include' directives)

See NixOS#144575.
Closes NixOS#243068.

nixos/doc/manual/release-notes/rl-2311.section.md

@@ -589,6 +589,8 @@ Make sure to also check the many updates in the [Nixpkgs library](#sec-release-2
   it's been unmaintained for several years and the author's website has
   vanished.
 
+- The `krb5` module has been rewritten, removing `krb5.libdefaults`, `krb5.realms`, `krb5.domain_realm`, `krb5.capaths`, `krb5.appdefaults` and `krb5.plugins`, which have been replaced by `krb5.settings`. Additionally, `krb5.config` has been replaced by `krb5.extraConfig`.
+
 - The `chrony` NixOS module now tracks the real-time clock drift from the
   system clock with `rtcfile` and automatically adjusts it with `rtcautotrim`
   when it exceeds the maximum error specified in