Accept Ed25519 identities in EXTEND2 cells

Once we have the link-connection part of prop220 implemented, we can start here, and allow relays to start accepting Ed25519 identities in their extend2 cells.

We'll have to update extend_cell_parse to handle these, and extend_cell_t to contain them.

We'll have to update channel_get_for_extend to look up by the complete set of link specifiers.

We'll have to update channel_connect_for_circuit to accept an ed25519 ID, if it hasn't already.

And we'll have to update circuit_extend to handle all that properly.