Add header for scan execution policy scanner dropdown

What does this MR do and why?

This MR resolves #413275 (closed)

Now Listbox allows select all in header

This MR adds header to scanner select

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Go to project or group
2. Secure -> Policies -> 'New policy' -> Scan execution policy
3. Choose scanner in Actions section

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.

Related to #413275 (closed)