Description of the validated prefixes format & the signature scheme

[ties]

I was looking at the signature scheme used by octorpki because I wanted to see how it was implemented (to see if it could be adopted for SLURM files). When doing so I could no easily find the implementation.

As a developer it may be helpful if both the data format as well as the signature format are more clearly described somewhere. The authoritative reference for the signature seems to be the implementation:

https://github.com/cloudflare/gortr/blob/fb7be39c6a6c4b0f1fff1733bd0f4f5bc77b2b02/prefixfile/prefixfile.go#L18

I'll propose documenting the output format for rpki-validator and possibly add a json schema. However, if an issue is made this may help prioritise the issue.