crypto/rsa: add EncryptOAEPWithOptions

crypto/rsa: add EncryptOAEPWithOptions

Co-authored-by: Filippo Valsorda <filippo@golang.org>
Change-Id: I78968794d609a7b343e5affc141d8ba96a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/722260
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Bypass: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>