TLS 1.3 defines a new SignatureScheme uint16 enum that is backwards
compatible on the wire with TLS1.2's SignatureAndHashAlgorithm. This
change updates the go testing code to use a single signatureAlgorithm
enum (instead of 2 separate signature and hash enums) in preparation for
TLS 1.3. It also unifies all the signing around this new scheme,
effectively backporting the change to TLS 1.2.

For now, it does not distinguish signature algorithms between 1.2 and
1.3 (RSA-PSS instead of RSA-PKCS1, ECDSA must match curve types). When
the C code is ready make a similar change, the Go code will be updated
to match.

[Originally written by nharper, tweaked significantly by davidben.]

Change-Id: If9a315c4670755089ac061e4ec254ef3457a00de
Reviewed-on: https://boringssl-review.googlesource.com/8450


Reviewed-by: David Benjamin <[email protected]>