[TLS] Updating for non-X.509 certificate types

[Eric Rescorla <[e-mail]>]

As noted in https://github.com/tlswg/tls13-spec/issues/722, the new fancy
TLS 1.3 Certificate structure doesn't map well to the various non-X.509
cert structures we have defined, specifically:

- Raw Public Keys
- Cached Info
- OpenPGP

Probably mapping each of these to 1.3 is relatively straightforward
(Raw public keys == a list with one key, Cached info == the hash of
each cert + its extensions, and so on), but I tend to think that given the
modest/specialized deployment of these extensions, it's better to do a
set of small bis RFCs to define each of these, rather than add a bunch
of clutter to TLS 1.3 proper.

Does anyone object to this? Volunteers.

-Ekr