Add conversion trait impls from `x509_parser::x509::X509Name` to `DistinguishedName`

[cyqsimon]

It would be very helpful for code that uses both x509_parser for verification and rcgen for generation.

The reverse is likely difficult/impossible because X509Name is a borrowed type so I haven't bothered to ask.

[djc]

I forget whether we already expose x509-parser in the public API, but one reason not to do this is to avoid having the dependency in public API, which then makes it impossible to bump its version without also bumping rcgen's version.

[cyqsimon]

I forget whether we already expose x509-parser in the public API, but one reason not to do this is to avoid having the dependency in public API, which then makes it impossible to bump its version without also bumping rcgen's version.

Man, the annoyances of maintaining a library 🙃. Yeah I concur this is a well-justified hesitation. I imagine this would be less of a problem when (and if) x509-parser reaches 1.0?

[djc]

I imagine this would be less of a problem when (and if) x509-parser reaches 1.0?

Probably, yes.