Add multicodec for DER-encoded X.509 certificate (#385)

* Add x509-certificate codec (#1)

* Bump x509-certificate type to 0x0210 to leave space for future CAR extensions

Co-authored-by: Rod Vagg <[email protected]>

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Co-authored-by: Rod Vagg <[email protected]>

Author: joaopenteado

table.csv

@@ -140,6 +140,7 @@ swhid-1-snp,                    ipld,           0x01f0,         draft,      Soft
 json,                           ipld,           0x0200,         permanent,  JSON (UTF-8-encoded)
 messagepack,                    serialization,  0x0201,         draft,      MessagePack
 car,                            serialization,  0x0202,         draft,      Content Addressable aRchive (CAR)
+x509-certificate,               serialization,  0x0210,         draft,      DER-encoded X.509 (PKIX) certificate per RFC 5280; single certificate only (no chain); raw DER bytes (not PEM)
 ipns-record,                    serialization,  0x0300,         permanent,  Signed IPNS Record
 libp2p-peer-record,             libp2p,         0x0301,         permanent,  libp2p peer record type
 libp2p-relay-rsvp,              libp2p,         0x0302,         permanent,  libp2p relay reservation voucher