Code Insights: Add code insights backend interface
Created by: vovakulikov
Closes part of https://github.com/sourcegraph/sourcegraph/issues/25874
Background
At the moment for the code insights pages, we already have some abstraction over API calls (query and mutation) here. But this API operates on a low level. It operates over setting cascade subjects and it clearly depends on the setting cascade. In order to be able to change API, we have to come up with something more abstract rather than working with setting cascade explicitly as we do in our existing API.
This PR solution
This PR just adds a common future Code Insights backend class (main interface and describing types of methods). In the next PRs, we will replace our old setting cascade API with a new implementation of that interface.
Note that this PR is the first of the next three PRs where we will replace API calls in our visual components.
Working draft PR with full changes of that API implementation you can find here https://github.com/sourcegraph/sourcegraph/pull/25858
Next steps
- Dashboard related pages migrations - working draft here https://github.com/sourcegraph/sourcegraph/pull/26151
- Code insights creation UI migration https://github.com/sourcegraph/sourcegraph/pull/26235