Ensure LDAP Group Sync by Filter sanitizes DNs

What does this MR do?

Fixes #8285 (closed).

This changes the LDAP Group Sync by filter feature and ensures that member DNs are normalized/downcases so comparison happens correctly.

This also adds in-memory caching of filter results. We've always done in-memory caching of other LDAP Group Sync search results to reduce the number of calls to LDAP. It was an easy add and brought everything into alignment.

Of note, the initial diagnosis from #8285 (closed) wasn't quite correct. All normalization prior to this change was actually done transparently because most LDAP search results are fed through the ...::LDAP::Group or ...::LDAP::Person class which does the normalization for us. This MR doesn't change that, but it also brings normalization into a common location to ensure that all DNs are normalized regardless of where they come from. For LDAP Group Sync by CN, this is redundant but for LDAP Group Sync by filter it's important. I think the change brings a lot of clarity, though.

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