This will verify that the signature of the ephemeral key used in the
DHE and ECDHE key exchanges is actually generated by the server.
This verification is done using the first certificate provided by the
server, however the validity of this certificate is not checked here.
Instead this code expects the validity to be checked earlier by
`TLSv12::handle_certificate`.
This add an implementation for the EMSA-PKCS1-V1_5-ENCODE function from
RFC8017 section 9.2. The verification of this encoding is implemented by
simply encoding the message to be verified, and then comparing the two
encoded string.
The digest info for the different hash function is from RFC8017 section
9.2 notes 1. These byte sequences are actually ASN.1 encoded data,
however these are always constant for a specific hash function and can
be treated as opaque byte sequences.