Flexible encoders for ML-DSA

Flexible encoders for ML-DSA

- Same UX as ML-KEM.  The main ASN.1 private key syntax is the one from
  Russ Housley's post on the LAMPS list, subsequently amended to tag the
  seed instead of the key (each of the three parameter sets will have a
  fixed size for the `expandedKey`):

    ML-DSA-PrivateKey ::= CHOICE {
      seed [0] IMPLICIT OCTET STRING SIZE (32),
      expandedKey OCTET STRING SIZE (2560 | 4032 | 4896)
      both SEQUENCE {
        seed OCTET STRING SIZE (32),
        expandedKey OCTET STRING SIZE (2560 | 4032 | 4896) } }

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26638)