crypto/internal/fips/ssh: implement SSH KDF

crypto/internal/fips/ssh: implement SSH KDF

For #69536

Change-Id: I0788469400de6a39cb22a6936a74d71066c476b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/626875
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>