crypto/internal/fips140/rsa: support separate MGF1 hash for EncryptOAEP
We might or might not want to expose it, but it makes the internal API
symmetrical, and lets us decide to do it in the future without changing
the FIPS module.
Updates #65716
Change-Id: Iea431a527ab17b9f00dee4da25761cedb2c2eba0
Reviewed-on: https://go-review.googlesource.com/c/go/+/630655
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Russ Cox <rsc@golang.org> Reviewed-by: Roland Shoemaker <roland@golang.org>