CMS signer identifier is incorrectly encoded

[Lukasa]

We have the following code for serializing and deserializing the subjectKeyIdentifier option in CMSSignerIdentifier:

swift-certificates/Sources/X509/CryptographicMessageSyntax/CMSSignerIdentifier.swift

Lines 58 to 62 in d8799da

	case .subjectKeyIdentifier(let subjectKeyIdentifier):
	try coder.serialize(
	ASN1OctetString(contentBytes: subjectKeyIdentifier.keyIdentifier),
	explicitlyTaggedWithIdentifier: Self.skiIdentifier
	)

swift-certificates/Sources/X509/CryptographicMessageSyntax/CMSSignerIdentifier.swift

Lines 39 to 45 in d8799da

	self = try DER.explicitlyTagged(
	node,
	tagNumber: Self.skiIdentifier.tagNumber,
	tagClass: Self.skiIdentifier.tagClass
	) { node in
	.subjectKeyIdentifier(.init(keyIdentifier: try ASN1OctetString(derEncoded: node).bytes))
	}

Both of these perform explicit tagging of the field. That's informed by this declaration from RFC 5652 § 12.1:

SignerIdentifier ::= CHOICE {
  issuerAndSerialNumber IssuerAndSerialNumber,
  subjectKeyIdentifier [0] SubjectKeyIdentifier }

However, this misses that the tags default for the document is specified as:

   DEFINITIONS IMPLICIT TAGS ::=

So the default tag is implicit unless one of the exceptions from ITU 680 § 30.6 applies. None of them do, so this should be implicitly tagged.