[ech] custom TLS client hello extensions

[Salz, Rich]

> Almost. The custom ext type would also need to be in the
inner CH (in compressed form) to get best interop I guess.

Perhaps we can ask on the TLS list what people are expecting. I think "custom extensions" are not widely used.  Maybe Matt has some feedback. I think, especially given OpenSSL's history, trying to anticipate all needs is a mistake. And then we're stuck with a misfit API, have to add "_ex" or worse "_ex2" functions and so on.
 
> I could envisage all sorts of PII being put in such
extensions, for example, and if that were the case, it
may well make sense to provide more fully featured
APIs. But if that doesn't happen, then probably better
to leave such for another day.

If none of the browsers use custom extensions and this doesn't support them in the first release, then it is hard to claim it's wrong.