feat: add pkix-pub and pkcs8-priv key codecs

Introduce container codecs for IETF-standard asymmetric key formats that carry the algorithm identifier alongside the key material, so implementers can reuse existing crypto libraries and specs rather than mint a per-algorithm codec for every new scheme.

- pkix-pub (0x40): SubjectPublicKeyInfo (SPKI) per IETF RFC 5280 section 4.1.2.7
- pkcs8-priv (0x41): OneAsymmetricKey (PKCS #8) per IETF RFC 5958 section 2
- both formats self-identify the key algorithm via the embedded AlgorithmIdentifier OID
- codes placed in the single-byte varint range (< 0x80) so existing users of libp2p-key (0x72) in peer IDs and IPNS names can migrate without paying an extra prefix byte per identifier
- 0x40 and 0x41 sit in the unclustered 0x39-0x4f free block, avoiding the ipld/CID-codec neighborhood at 0x70-0x7f

Refs:
- https://www.rfc-editor.org/rfc/rfc5280
- https://www.rfc-editor.org/rfc/rfc5958
- libp2p/specs#711

Author: lidel

table.csv

@@ -36,6 +36,8 @@ dns,                            multiaddr,      0x35,           permanent,
 dns4,                           multiaddr,      0x36,           permanent,
 dns6,                           multiaddr,      0x37,           permanent,
 dnsaddr,                        multiaddr,      0x38,           permanent,
+pkix-pub,                       key,            0x40,           draft,      DER-encoded ASN.1 type SubjectPublicKeyInfo (SPKI) according to IETF RFC 5280 section 4.1.2.7; public key algorithm identified by the embedded AlgorithmIdentifier OID
+pkcs8-priv,                     key,            0x41,           draft,      DER-encoded ASN.1 type OneAsymmetricKey (PKCS #8) according to IETF RFC 5958 section 2; private key algorithm identified by the embedded AlgorithmIdentifier OID
 protobuf,                       serialization,  0x50,           draft,      Protocol Buffers
 cbor,                           ipld,           0x51,           permanent,  CBOR
 raw,                            ipld,           0x55,           permanent,  raw binary