Add a backflow merge policy check

[premun]

Context

For backflows, we need to verify that people haven't messed up the <Source> tag in the Version.Details.xml as that would break backflow.

Goal

• Create a new backflow merge policy which will run on source-enabled subscriptions from the VMR
  • The policy will check if the BAR ID and VMR SHA in the <Source> tag of the Version.Details.xml file in the PR head branch matches the currently flown build
  • It needs to refresh for every commit
  • In case of failure, it needs to explain why, give people instructions / contact for our team
  • The policy will probably do more in the future
• Include the policy in the Standard merge policy

Notes

Should be done together with #4526 (not sure which first)