Add linked file mode for yaml code block action

What does this MR do and why?

This MR adds linked file mode

UI for linked file mode for in custom action for scan execution policy

Till this conversation and design are finalised, this will be initial state of linked file mode. It is behind ff.

Will be extended, when conversation is resolved.

Screenshots or screen recordings

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

Description	UI
file path and validation	file with validation.mov
Existing file	existing path and delete.mov

How to set up and validate locally

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

 Feature.enable(:compliance_pipeline_in_policies)

1. Go to a project or group
2. Secure -> Policies -> New policy -> Scan execution policy
3. Add new action -> Execute a YAML code block
4. Select option Linked existing CI file
5. Type in file path, remove it, type switch back and forth between rule mode and yaml mode

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 #428507 (closed)