Add approvers select dropdown

What does this MR do and why?

Add policy action builder into scan result policy page. There is another MR in parallel to this one dealing with policy rules. This is applicable to both new and existing policies.

It is important to mention that although the YAML supports multiple attributes in regards to approvers, rule mode will prioritize converting them to ids.

Follow up from: !80221 (merged)

Related issue: #351166 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable scan_result_policy feature flag
2. Create a security project as described in the docs
3. Create a new scan result policy with the project owner

MR acceptance checklist

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