DAL/Incentives: add new `dal_participation` RPC

This RPC should give information about the current participation of a delegate: in particular the number of attested slots in the current cycle.

A similar RPC exists for participation in consensus (for consensus attestations), see src/proto_alpha/lib_protocol/delegate_missed_attestations_storage.ml, function For_RPC.participation_info. A similar structure can be used for the DAL.