Implement connector builder server

[lmossman]

Tell us about the problem you're trying to solve

As described in the Tech Spec, a connector builder server will be implemented to back the UI by providing it endpoints to fetch stream information and test read commands.

Describe the solution you’d like

The connector builder server is defined by the OpenAPI spec (from #17424), and the python API skeleton implementation will be auto-generated from that spec (see #17813).

See OpenAPI spec here for the full details on the endpoints: #17535

This ticket is about actually implementing those server endpoints. The /stream/read endpoint will use the python CDK method implemented in #17542 to fetch the relevant data from the source API, and then return that data in the correct format.

The /streams/list endpoint will implement some logic to parse the connector definition and return the relevant stream information.

• Note: the URLs returned in this endpoint can just keep the templated values in the URLs, since resolving these without actually making requests may not be doable

Acceptance Criteria

• The connector-builder-server endpoints are implemented as defined by the OpenAPI spec