refactor(storage): type-safe access to single fields of peer

Every field of Peer now has a corresponding unit struct defined in the peer_field module, which implements the PeerFieldDef trait which in turn contains information about the field's name, tarantool type and rust type.

This information currently is used in the field_by_raft_id method and is generated by the define_peer_fields macro, which also automates the generation of the format (peer_format function) for the "raft_group" space.