Add API endpoint to create a new RFD

[benjaminleonard]

Helpful for a couple reasons; the first being we can automatically reserve the next available RFD number without worrying about clashes, the second is that we can create RFDs directly from the RFD site—which apart from being more intuitive means we can validate the document attributes are set up correctly.

Related to: https://github.com/oxidecomputer/rfd/issues/524