a construct that's both simple and extensible(?)

[bc-pi]

Although we can definitely punt on it for now, I think we could choose a construct that's both simple and extensible.

"signed_headers": {
   "txn-token": "txn token hash",
   "x-custom-context": "x-custom-context hash"
}

Unfortunately we need the WPT to sign miscellaneous stuff, because we don't want a new WIT for every call. Having said that, as far as I can tell it does NOT need to sign the WIT itself.

Originally posted by @yaronf in #19 (comment)

[bc-pi]

#19 (comment) related followup in the aforementioned comment

[yaronf]

Won't do, too complex.