[Rails] Add additional context type for Codebase as Chat Context to API

Problem to solve

We are adding a new context type, "repository", to the available contexts in Duo Chat. This is part of the Codebase as Chat Context. Ultimately, this will be code pulled from a given repository using semantic search.

A necessary first step is to add the new context type (include_repository_context on the backend). When received from API clients, this context type will have an empty string as its content. Later work will enable the backend to replace the empty string with the retrieved code context.

Proposed solution

Add the include_repository_context context type. Put it behind duo_include_context_repository feature flag.