crypto/internal/fips/drbg: add CAST and service indicator

crypto/internal/fips/drbg: add CAST and service indicator

For #69536

Change-Id: If216a7537013bd9bbe90026ffe983a6bf50029c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/625435
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>