crypto/internal/fips/mlkem: implement CAST, PCT, and service indicator

crypto/internal/fips/mlkem: implement CAST, PCT, and service indicator

For #69536

Change-Id: Id9d2f6553ab006d0d26986d22a4a756b9cf1bf71
Reviewed-on: https://go-review.googlesource.com/c/go/+/626936
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>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>