This seems like something that would likely be useful in general for people writing Config.GetCertificate functions.

We kind of have an inverse version of this already with ClientHelloInfo.SupportsCertificate, but that requires you already have a certificate to test against, which is obviously not the case in this case and the autocert case (hence why it had to be written in the first place).

I'd not be opposed to adding the following method in crypto/tls:

// SupportsECDSA returns true if the ClientHelloInfo indicates the the client supports both ECDSA based
// key exchanges and cipher suites.
func (*ClientHelloInfo) SupportsECDSA() bool

cc @FiloSottile