Add yaml conversion to vulnerability management policy editor

What does this MR do and why?

Related #465821 (closed)

Add yaml conversion to vulnerability management policy editor

This adds basic conversion between yaml and policy object to the editor component for the vulnerability management policy type. It also parses basic errors and starts with a simple default yaml configuration.

MR acceptance checklist

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Screenshots or screen recordings

vulnerability-management-policy-yaml

How to set up and validate locally

1. enable feature flag vulnerability_management_policy_type
2. go to a project
3. go to Secure > Policies
4. click New policy
5. click Select policy on the Vulnerability management policy card
6. validate it shows a yaml preview with type: vulnerability_management_policy, name: '', description: '', and enabled: true included.