ASN.1 format tagging seed, key now octet string

ASN.1 format tagging seed, key now octet string

- 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 (64),
      expandedKey OCTET STRING SIZE (1632 | 2400 | 3168)
      both SEQUENCE {
        seed OCTET STRING SIZE (64),
        expandedKey OCTET STRING SIZE (1632 | 2400 | 3168) } }

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/26639)