Extend the API to extract selected_fields of a connection with the v1/connection path

[jonseymour]

A relatively recent change introduced the ability to set selected fields via the API (ref: #35798 )

However, the API currently lacks the ability to extract selected fields via the matching GET API

This is suboptimal because it means that round-tripping of connection definitions through API is irretrievably lossy - you can CONFIGURE selected fields with the API but you cannot DISCOVER selected fields with the API.

As a result, anyone wishing to maintain connection definitions for use in terraform (for example) is forced to reference schema data from external sources in order to create an artefact that can be used to set the selected fields via the api &/or terraform.

As it stands, it is not possible, for example, to configure selected fields in the airbyte UI and then export the configured fields via the API because as it stands, the API doesn't support this use case. Again, it is presumably possible to dig into airbyte's relational schema, but any need to do so indicates a glaring deficiency in the API itself