third-party verification api

It'd be cool if keybase supported some sort of API for allowing sites to support verification.

Keybase would have a verification option where you supplied your username, and a link to the site's location for the api. Keybase would query that api with the username, and the site would return the user's proof. This way, keybase doesn't have to implement each site manually, instead allowing the sites themselves to support keybase.

The query would look something like this:
`GET https://domain.name/kbapi/proof/`

to which the site would respond with the same file that would normally go in `.well-known`.

Because the api would not require any authentication (it only serves static content), the proofs would be available to everyone.

Start by reviewing the proposed GET /kbapi/proof/ endpoint and the existing .well-known proof format described in the issue. Define the API contract, verification behavior, and integration scope; done requires an agreed design because no implementation files or tests are identified.

Written by the indexing model from the issue text.