[Semantic Code Search] Duo Chat Integration - Backend
## Context
Develop backend services to support codebase as context for Duo Chat.
## Proposal/Issues
### Milestone 1 - Basics
**Backend/Frontend integration**
- https://gitlab.com/gitlab-org/cloud-connector/gitlab-cloud-connector/-/issues/69+
- https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/922+
- https://gitlab.com/gitlab-org/gitlab/-/issues/536906+
**Backend**
For the order of implementation on the Rails/AIGW side would be:
1. https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/903+
2. https://gitlab.com/gitlab-org/gitlab/-/issues/537896+
3. https://gitlab.com/gitlab-org/gitlab/-/issues/537897+
We will need to add a unit primitive for the "Semantic Search Tool" (see https://gitlab.com/gitlab-org/cloud-connector/gitlab-cloud-connector/-/issues/72). This can be done any time before Step 3 above.
### Milestone 2 - Directory filters, Tracking, Projects API
- https://gitlab.com/gitlab-org/gitlab/-/issues/548423+
- https://gitlab.com/gitlab-org/gitlab/-/issues/548421+
- https://gitlab.com/gitlab-org/gitlab/-/issues/547112+
epic