Filter user groups for LDAP provider

**Is your feature request related to a problem? Please describe.**
We have a LDAP source connected with close to 100k users. Now some of these users get a certain group which allows them to login at some servers. It would be nice to be able to add a filter to the ldap provider only looking at users with that group. That would drastically improve the performance I guess.

Right now our authentik-server takes all the cores that are available (for testing 32) to serve the ldap providers request. After some time the server gets very unresponsive.

**Describe the solution you'd like**
LDAP provider with to filter a group of users that are provided by the provider

**Describe alternatives you've considered**
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**Additional context**
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