Wish: Make federation configurable for subsets of users

This issue has been migrated from [#11041](https://github.com/matrix-org/synapse/issues/11041).

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I'd like to enable/disable federation for users based on their LDAP identity or more detailed e.g. on groups they are assigned to in LDAP.

In a first step it would be very nice to have some kind of a whitelist/blacklist allowing/disallowing users from using the federation feature analog to whitelist/blacklisted servers.