Add `scan_result_policy` as an approval rule functionality
<!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION --> *This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.* <!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION --> ### Implementation plan 1. ~backend Add a feature flag which will be handling this feature only. **Note** that the feature flag won't be a switch between the existing security approval logic (Vulnerability-Check) and this upcoming feature. 1. ~backend Add `scan_result_policy` as a new type into the existing logic for `AVAILABLE_POLICY_TYPES`. Including the addition of the logic/methods to facilitate access to variables which will be used for security approvals. 1. ~backend Based on the implementation of the previous item, those methods can be used and added into the MR approval logic (in addition to the existing logic not as an either or). 1. ~documentation update the documentation by adding the new policy type with the description of the new yaml fields and examples. 1. With the above implemented we would be in a good position for removing the feature flag and to deprecate the existing vulnerability-check. Pending issues related to the ~frontend
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