Support building `DistinguishedName` from RFC 4514 input

[natevw]

It would be helpful if DistinguishedName (and perhaps its subcomponents?) could support initialization from RFC 4514–formatted strings.

RelativeDistinguishedName.Attribute and its .Value have a rough sort of reverse form of this in

swift-certificates/Sources/X509/RDNAttribute.swift

Lines 211 to 233 in a846fc7

	public var description: String {
	let attributeKey: String
	switch self.type {
	case .RDNAttributeType.commonName:
	attributeKey = "CN"
	case .RDNAttributeType.countryName:
	attributeKey = "C"
	case .RDNAttributeType.localityName:
	attributeKey = "L"
	case .RDNAttributeType.stateOrProvinceName:
	attributeKey = "ST"
	case .RDNAttributeType.organizationName:
	attributeKey = "O"
	case .RDNAttributeType.organizationalUnitName:
	attributeKey = "OU"
	case .RDNAttributeType.streetAddress:
	attributeKey = "STREET"
	case let type:
	attributeKey = String(describing: type)
	}
	return "\(attributeKey)=\(value)"
	}

and https://github.com/apple/swift-certificates/blob/a846fc71499526eeceae5e3b739235ff249d4433/Sources/X509/RDNAttribute.swift#L148C46-L150 and this is highlighted in the docs

swift-certificates/Sources/X509/Docs.docc/Examining Certificates.md

Lines 65 to 66 in a846fc7

	A string representation of a ``DistinguishedName`` can be obtained by using `String(describing:)`. This
	uses the common [RFC4514 format](https://www.rfc-editor.org/rfc/rfc4514) for textual ``DistinguishedName``s.

as following the standard, but there is currently no parsing/deserialization offered in the other ddirection

[Lukasa]

Yeah, this sounds like a reasonable feature. I'd be happy to see someone take a swing at implementing this.