crypto/ecdsa: add low-level encoding functions for keys

crypto/ecdsa: add low-level encoding functions for keys

Fixes #63963

Change-Id: I6a6a4656a729b6211171aca46bdc13fed5fc5643
Reviewed-on: https://go-review.googlesource.com/c/go/+/674475
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>