Add new get_many_to_many XRPC endpoint

Added a new XRPC API endpoint to fetch joined record URIs, termed get_many_to_many (we talked about this briefly on Discord already). It is implemented and functions almost identical to the existing get_many_to_many_counts endpoint and handler. Some of its possible flaws like the two step lookup to verify a matching DID is indeed active are duplicated as well. On the plus side, this should make the PR pretty straightforward to review and make it easier to modify both endpoints later on when a more efficient way to validate the status of DIDs is possible. If you have comments remarks etc. I am happy to work on some parts again.