Write OpenAPI spec for the Connector Builder backend server

[lmossman]

Tell us about the problem you're trying to solve

The API for the Connector Builder backend server needs to be defined and approved, so that development on both the frontend and backend pieces that rely on the API can begin.

Describe the solution you’d like

Write out an OpenAPI spec for the Connector Builder server, so that typescript client and python server code can be auto-generated from the API spec.

The spec should address the requirements stated in the Tech Spec

Acceptance Criteria

• An OpenAPI spec for the Connector Builder server that solves the phase 1 requirements is approved by the Connectors Extensibility team