crypto/tls: add support for Certificate Transparency TLS extension

[titanous]

I'd like to be able to serve SCTs via the TLS extension with crypto/tls, similar to how the Certificate struct supports an OCSPStaple. I'm happy to put it together and submit a CL if the feature would be accepted.

/cc @agl

[rsc]

Adam, does this sound reasonable to you? Thanks.

[agl]

Yes, this is reasonable and very similar to OCSP stapling that's currently supported.

[titanous]

https://go-review.googlesource.com/8988

[gopherbot]

CL https://golang.org/cl/8988 mentions this issue.