Add multicodec for DER-encoded X.509 certificate

[joaopenteado]

This PR proposes a new multicodec entry for the canonical binary representation for X.509 certificates, as defined in RFC 5280, encoded in DER form.

Notes

• This covers exactly one Certificate object; if there’s demand for transporting multiple certs, CRLs, OCSP responses, or PKCS#7 bundles, those should be proposed as separate, unambiguous entries.
• Version is not restricted to v3 — v1 and v2 certificates are valid encodings and will parse under the same ASN.1 type.
• Consumers can unambiguously decode and parse the payload knowing only the multicodec code.

[rvagg]

sgtm