Indicate the signature mechanism for the API

[earth2marsh]

As we relax the idea that paths must be unique, it can complicate the signature evaluation process. Consider adding a signature style as part of the info block, such as path+verb, action-header, or similar? (haven't thought this through fully, I just wanted to stub out the idea.)

[captainsafia]

I came across this discussion item while reasoning through what role operation IDs play in the v4 spec. Is the idea that these signatures would replace the existing operation ID property?

[earth2marsh]

We discussed signatures on today's call. Video will be posted here: #107 (comment) (and see also the doc mentioned in the comments)