Implement client certificates for TLS 1.3 in Go.
Tested by having client and server talk to each other. This adds the certificate_extensions field to CertificateRequest which I'd previously missed. (We completely ignore the field, with the expectation that the C code won't have anything useful to do with it either.) Change-Id: I74f96acd36747d4b6a6f533535e36ea8e94d2be8 Reviewed-on: https://boringssl-review.googlesource.com/8710 Reviewed-by: David Benjamin <[email protected]>