[p3t] Simple Cone Search starting point added

[Russ Allbery]

I have added a starting point for looking at an OpenAPI schema for Simple
Cone Search. README is available at:

    https://github.com/ivoa/PTTT/tree/main/cone-search

This is mostly a framework to get started and let us quickly iterate on
the OpenAPI schema by letting FastAPI do most of the work. I focused on
getting the framework in place rather than adding a lot of API details.

This provides only a sync GET interface without capabilities or
availability. For a more fully-fleshed example, we'll want to add an async
interface with UWS and models for capabilities and availability. I plan on
working on that next if this is a suitable starting point.

Some additional notes and cautions:

* FastAPI is not an ideal OpenAPI schema generator, and I suspect it will
  not generate exactly the schema that we would want. Please consider this
  only a proof-of-concept for fast iteration. An OpenAPI schema suitable
  for publication would likely require some additional manual tweaking.

* FastAPI makes documentation available with both Swagger and Redoc. Both
  of them have their quirks, but they're both useful renderers for human
  browsing. I personally prefer Redoc, but I encourage folks to try
  both. Swagger has a bug that causes it to double-escape the example
  VOTable response. This appears to be rendered correctly in Redoc.

* Apologies for the giant blob of XML that's awkwardly encoded in the JSON
  file. I wanted to show how XML responses can be documented, but the
  resulting JSON is pretty ugly.

* The error model is a quick hack based on the errors that FastAPI
  produces by default, including the 422 error code that FastAPI likes to
  use for requests that fail schema validation. This is probably not the
  exact error model that we want. I threw it in there so that people could
  see how we could define structured errors, not to propose this specific
  structured error format as the best one (although it is convenient in
  Python since FastAPI generates it by default). We will probably also
  want to document some additional error codes (at least 401 and 403).

Let me know if you have any comments, questions, or problems following the
README.

-- 
Russ Allbery (eagle at eyrie.org)             <https://www.eyrie.org/~eagle/>