Update secret (Backend)

Why are we doing this work

This adds a GraphQL mutation to the backend, so that the frontend can update secrets.

Design

Relevant links

• Frontend issue: Frontend: Allows users to update secrets (#470398 - closed)

Non-functional requirements

• Documentation: update GraphQL API doc
• Feature flag: behind secrets_manager flag
• Performance:
• Testing: specs for new GraphQL mutation

Implementation plan

• GraphQL mutation - !188740 (merged)
  • Take into consideration the configured permissions for the current user's role.

Verification steps

TODO