Chrome uses the platform certificate verifier and thus cannot reliably
expect PSS signatures to work in all configurations. Add an API for the
consumer to inform BoringSSL of this ability. We will then adjust our
advertisements accordingly.

Note that, because TLS 1.2 does not have the signature_algorithms_cert
extension, turning off TLS 1.3 and using this API will stop advertising
RSA-PSS. I believe this is the correct behavior given the semantics of
that code point.

The tests check the various combinations here, as well as checking that
the peer never sends signature_algorithms_cert identical to
signature_algorithms.

Bug: 229
Change-Id: I8c33a93efdc9252097e3899425b49548fc42a93a
Reviewed-on: https://boringssl-review.googlesource.com/27488
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Steven Valdez <svaldez@google.com>