Support an API that accepts Certificate Signing Requests

[di]

Description
Currently the /api/v1/signingCert API accepts a JSON body that includes a public key (publicKey) and a signed email address (signedEmailAddress).

Some ecosystems (e.g. Python) have good primitives for generating Certificate Signing Requests, and it would be preferable for Fulcio to support an API that accepted CSRs directly as an alternative to the current Fulcio-specific request body when requesting a signing certificate.

[haydentherapper]

@dlorenc @lukehinds - I'm taking a look at adding support for CSRs as a key delivery and proof of possession mechanism. It was mentioned to me that there was a conversation from awhile ago around not supporting this - Do you recall the context?

I'm in support of this feature, as there's more widespread support for CSR generation than encoding public keys. It also simplifies the proof of possession challenge, as the CSR is self-signed.

[dlorenc]

I have no problems adding it for compatibility.