Extend the SERVERINFO file format to include an extensions context

This enables us to know what messages the extensions are relevant for in
TLSv1.3. The new file format is not compatible with the previous one so
we call it SERVERINFOV2.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3298)