x509-cert: `BMPString` support for `DirectoryString`

[Chaoses-Ib]

der already supports BmpString according to #693 (comment), but DirectoryString in x509-cert still doesn't:

formats/x509-cert/src/ext/pkix/name/dirstr.rs

Lines 39 to 55 in 217b6ef

	/// The implication of the above paragraph is that `PrintableString` and
	/// `UTF8String` are the new types and the other types are legacy. Until
	/// the need arises, we only support `PrintableString` and `UTF8String`.
	///
	/// [RFC 5280 Section 4.2.1.4]: https://datatracker.ietf.org/doc/html/rfc5280#section-4.2.1.4
	#[derive(Clone, Debug, Eq, PartialEq, Choice, ValueOrd)]
	#[allow(missing_docs)]
	pub enum DirectoryString {
	#[asn1(type = "PrintableString")]
	PrintableString(PrintableString),
	#[asn1(type = "TeletexString")]
	TeletexString(TeletexString),
	#[asn1(type = "UTF8String")]
	Utf8String(String),
	}

BmpString is still used by Windows when creating certs with non-ASCII strings. The lack of support for BmpString makes x509-cert unable to read non-ASCII names from those certs.

Sample cert:

-----BEGIN CERTIFICATE-----
MIIDSDCCAjCgAwIBAgIQKPABcjfrcIpG1K6NEhSpIjANBgkqhkiG9w0BAQUFADBN
MUswSQYDVQQDHkIAVwBEAEsAVABlAHMAdABDAGUAcgB0ACBt92yMACwAMQAzADMA
OQAwADYANwAxADYAMwA5ADAAOAAzADMAMQA5ADMwHhcNMjUwNTAyMTUwMDM5WhcN
MzUwNTAyMDAwMDAwWjBNMUswSQYDVQQDHkIAVwBEAEsAVABlAHMAdABDAGUAcgB0
ACBt92yMACwAMQAzADMAOQAwADYANwAxADYAMwA5ADAAOAAzADMAMQA5ADMwggEi
MA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQDlgz7PDdlf6UzTtcWMRpaGGCdg
KYQOBOJWg6lnTQMxALfWD2bzVrbLM8N2HlMUi1AhDmr3EX6xgFi3gdXB9DgZKwEB
C1yq9CtU3QEyb/BKdsSaBasZPwfNEak8shAacvi0ssDhd05CIPYpvAlyO1RP1+m9
g3LaHaTYnlPM1PYGr3yJ8qMprGsMepgrLf1AlHQRhWyEml5C0s/1WaflYo/NJnBy
thyxksCEM2+AeO8A6CtwkL1yL47i6WImqk68NbeDiYalGwFt1Nb1Yd8PVbE7FT36
4vdMJ7jdQxzsCDiIq+KORZGSF/nVVpDeWRy8W0YtcfQ0LRbpIdEpRJOpK2UZAgMB
AAGjJDAiMAsGA1UdDwQEAwIEMDATBgNVHSUEDDAKBggrBgEFBQcDAzANBgkqhkiG
9w0BAQUFAAOCAQEAycUVQ5SKUGsdQL5hkvJchs5I43/zEfEHRaT75lYlMn2lAMsa
qX4vDBanWs0no9+0/7j0hPRBamNZGFThE6MswXcYdJrxhfp0ZjHntK0IMWO8riVO
Y6O/4xHF8rUPQkOBFIcRBF19Ads2Qdfgtjzc+288YRCLiZ4TmWjYlg9TVst7x/Fo
Hl1xH2Nbvm8AYPX9iPoqEzbq9aPy2MWi9Pf+R0lfqAjsdvj85A4dZKD2ovjOYzPX
PBSJQizmCHjovucot2iB5MumoSGloHbIFZl2fe/2jqDjPwX4wDoYO94kenlhxedo
lX4Tf6VRXWA0hVpJiE7NQMaVtO6rO/5P0pE1Pg==
-----END CERTIFICATE-----