Send ed25519 IDs in EXTEND2 cells
Once we have #19301 (moved) complete, we can update clients to generate circuits using ed25519 identities.
We'll have to actually store the ed25519 identity in the extend_cell_t when we make in .
We'll have to encode it as part of extend_cell_format.
We'll have to decide whether to use it: it's only okay to send the ed25519 ID when both servers support the new link handshake.
We can enable this with a tristate, for testing, and to make sure that this turns on for a big pile of clients at once.