Add a forward flow merge policy check

[premun]

Context

For forward flows, we need to verify that people haven't messed up the source-manifest.json record in the VMR.

Goal

• Create a new forward flow merge policy which will run on source-enabled subscriptions into the VMR
  • The policy will check if the BAR ID and VMR SHA in the source-manifest.json 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 #4525 (not sure which first)

[adamzip]

@premun I assume this should work for batched PRs as well - when the policy merge check triggers, it should gather all the flowed builds contained in the PR and check the corresponding entries in source-manifest.json