crypto,crypto/x509: implement MessageSigner

And use it in crypto/x509. This allows people to implement single-shot
signers which do the hashing themselves.

Fixes #63405

Change-Id: I038c2e10f77b050b6136c4c0a5b031cb416f59aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/654375


Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>