DAL/Node: new get peers RPCs

What

Add two new RPCs get_slot_indexes_peers and get_pkhs_peers, which are similar to get_topics_peers, but do not list the peers per topic, but instead by slot index and by pkh, respectively.

Why

get_topics_peers is too verbose, as for an operator (and also bootstrap) profile there are too many topics (32 * number of attesters).

Manually testing the MR

In the only test using the get_topics_peers I've manually added calls to the new RPCs and checked the output.