crypto/tls: Expose signature_algorithms in ClientHelloInfo

[titanous]

The signature_algorithms extension is provided in the TLS 1.2 ClientHello and could be used with the existing GetCertificate hook to choose between certificate chains with RSA and ECDSA signatures if exposed in ClientHelloInfo.

/cc @agl

[agl]

You don't need to look at signature_algorithms because ClientHelloInfo already includes the offered cipher suites, curves and point formats.

[agl]

(To elaborate, TLS cipher suites specify the certificate format too. So an …_ECDSA_… cipher suite has to use an ECDSA certificate. The crypto/tls code is smart enough not to try and select an RSA cipher suite if you hand it an ECDSA certificate.)

[titanous]

Makes sense, thanks!