Add main body for scan result policy rule mode

What does this MR do and why?

Add main body for scan result policy rule mode to be further extended with policy rule and action builders.

Follow up from !80029 (merged)

Related issue #351166 (closed)

Screenshots or screen recordings

Existing YAML mode still being the default but allowing rule mode option

Rule mode body with name, description and enabled and place holders for rule and action builders

Disabled fields when YAML fails to parse

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

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.