Allow injecting `searchBase` and `searchFilter` via a SecretClass in ldap AuthenticationClass

A customer reached out about enabling dynamic searchBase and searchFilter configuration of AuthenticationClasses for LDAP.

They are currently deploying NiFi clusters per team, that use a central ldap provider for authentication. Every team has an individual bind user that is only allowed to access ldap structures which are relevant for that team - which means that every team needs an individual 'searchBase' setting.

For the bindUser setting, we allow to configure that per namespace via a secretclass, so this is not an issue.

For the searchBase, this is hardcoded in the AuthenticationClass as a String, which at the moment means that the user has to have a dedicated authenticationclass per group that targets the same ldap.

One possible solution could be to allow specifying the searchBase and searchFilter via a SecretClass as well, this would allow the admininstrator in charge of maintaining the SecretClass to optionally delegate the searchBase (and searchFilter) to the users of this AuthenticationClass. Since these SecretClasses can be scoped to look in the namespace of a pod starting up, this would allow configuring a different searchBase per namespace, instead of just one global one.

An idea for how this could look in the CRDs is shown below, but this would most probably be breaking..

---
apiVersion: authentication.stackable.tech/v1alpha1
kind: AuthenticationClass
metadata:
  name: ldap-simple
spec:
  provider:
    ldap:
      hostname: my.ldap.server
      port: 389
      searchBase:
        secretClass: openldap-searchbase
      bindCredentials:
        secretClass: openldap-bind-credentials
---
apiVersion: secrets.stackable.tech/v1alpha1
kind: SecretClass
metadata:
  name: openldap-searchbase
spec:
  backend:
    k8sSearch:
      searchNamespace:
        pod: {}
---
apiVersion: v1
kind: Secret
metadata:
  name: my-admin-credentials
  namespace: userns1
  labels:
    secrets.stackable.tech/class: openldap-searchbase
stringData:
  searchBase: blablabla

The need for this in this specific case arises out of the organizational practice of not having a global ldap bind user but rather "team specific" bind users that cannot access user objects from different teams.
An organizational solution for this issue would be to change this - but that is not always possible, so the question would be if making this configurable is something we want to add.

The payload names no implementation files or tests. Start by locating the AuthenticationClass LDAP CRD and the SecretClass resolution path; compare existing bindCredentials handling with the proposed searchBase and searchFilter fields, then define CRD compatibility and tests proving namespace-scoped values are used.