proposal: x/crypto/acme/autocert: export supportsECDSA somewhere?

[bradfitz]

Proposal Details

I recently found myself needing x/crypto/acme/autocert's func supportsECDSA but it's not exported.

Copy/pasting it works, but is slightly sad as it's kinda long and I worry about it getting out of sync.

It's possible to use it indirectly via ugly hacks like https://github.com/tailscale/scertec/pull/3 (using a fake acme/autocert.Cache that looks which keys are accessed) but it's a bit too ugly.

Proposal: export that code somewhere? method on ClientHelloInfo even?

/cc @rolandshoemaker