Expose `alg` on PublicKey

[rickvanprim]

Presently the PublicKey field exposed in CertificateSigningRequestParams doesn't allow you to do anything with it aside from comparing it to another instance of PublicKey. It would be nice to be able to check which algorithm was used to generate the PublicKey.

In my use case, I control both the client side generating the CSR and the server side signing it, so I think it makes sense for the server to check that the algorithm used in the CSR's PublicKey matches what is expected.