[prop220] send CERTS cells correctly for Ed25519
Part of implementing proposal 220 is to make sure that severs receiving connections send all the new certificate types, including:
- identity->signing cert (type 4)
- signing->TLS-RSA (6) (respondor only)
- signing->link (5) cert (initiator only)
- RSA identity cross-cert (7)
This code is mostly implemented in my WIP legacy/trac#15055 (moved) branch.
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