PoC: Separate Access Model for Deployment Approval (Base Modeling)

What does this MR do and why?

This MR is a PoC that represents the backend implementation for Deployment Approval Rules. It's for giving a high-level overview of the entire implementation. This is NOT FOR MERGE.

Related #345678 (closed)

The approach

So the idea is to introduce the ProtectedEnvironment::ApprovalRule model to define who can approve/reject a deployment. The functionality is identical with ProtectedEnvironment::DeployAccessLevel model, except the following points:

• ProtectedEnvironment::ApprovalRule has FK to Deployments::Approval, meaning we record "who signed-off in which rule". This is useful to track which rule doesn't have sufficient approval.

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How to set up and validate locally

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MR acceptance checklist

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