[BE] Support `fallback_behavior` for approval policies

We want to introduce the fallback_behavior to merge request approval policies as outlined in &10816 (closed):

The idea for MVC is quite simple:

• we add toggle to YAML mode only,
• with the toggle enabled (potentially something like fallback_behavior: { fail: closed }) we will behave as we are right now,
• with the toggle disabled (potentially something like fallback_behavior: { fail: open }) we will not require approvals when policy or project is misconfigured,
• we use fallback_behavior: { fail: open } format to allow for further extensions in the future, potentially we could have a list of scenarios where we want to have different scenarios or some exceptions, like we had for many previous features
• the fail open option can be defined per policy.