Active Context: Add API endpoints to support chatops commands

Description

We want to support some chatops commands to manage Active Context connections and options. We cannot use C2 change requests for this purpose, which also brings independence in managing our Active Context infrastructure.

This adds the following API endpoints to support chatops commands listed in #565395 (closed):

• GET /api/v4/admin/active_context/connections
• PUT /api/v4/admin/active_context/connections/activate
• PUT /api/v4/admin/active_context/connections/deactivate
• PUT /api/v4/admin/active_context/collections/:id
• PUT /api/v4/admin/active_context/code/enabled_namespaces

After this merge request, we will implement the necessary tooling in the Chatops project.

Related to #565395 (closed)