Create new secret (Backend)

This is the backend issue for allowing users to create a new secret.

Frontend issue: #470385 (closed)

Design

What should be done here

• GraphQL mutation
  • Take into consideration the configured permissions for the current user's role.
  • We can split this into multiple MRs, like doing the environment and branch scoping in their own separate MRs.
  • The secret value will be stored in OpenBao under the secrets engine of the project.
  • Other information/metadata of the secret will be persisted in the database.
  • Given we only want barebones functionality for the closed experiment, no scoping, expiration, and rotation fields needed yet.