WCCE-RS: Specify default CSR subject DN

[lvkv]

wcce-rs currently doesn't support supplying a subject name in PKCS #10 certificate signing requests. This is because a large number of 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).

Because of this, wcce-rs uses a placeholder subject name when serializing a CSR using the rustls/rcgen library. However, we don't override the default DN provided by the library, which leads to obviously incorrect certificate authority logs like this:

The request was for
               CN=rcgen self signed cert.

This PR updates the library and documentation to use a more sensible default.

wcce-rs should also grow support for supplying subject names in the request. I've created an issue for that over at #4.