Support subject name specified in CSR

[lvkv]

wcce-rs currently doesn't support supplying a subject name in PKCS #10 certificate signing requests. This is because many AD CS certificate templates instead supply that value using AD attributes—essentially discarding the value supplied by the CSR in the process (i.e., the template has Subject Name > Supply in the request unset).

However, there are use cases for this, and some users might be surprised by wcce-rs's lack of support for the seemingly basic feature.

The options I see here are:

• Add a builder interface for distinguished names, either a homegrown one or, since this part of the codebase is already a thin layer over rustls/rcgen, plumb up rcgen's already great DN builder
• Keep it simple and parse a single string into a DN and assume UTF-8 encoding for everything