FE: Introduce scan execution strategy for Scan Execution Policies

Overview

This issue implements a new UI component for Scan Execution Policies that simplifies configuration for MR approval policy compatibility, as shown in this design.

Problem

Currently, users must understand complex GitLab pipeline mechanics to configure Scan Execution Policies that work properly with MR approval policies. This leads to confusion and often results in users selecting "all branches" (*), causing unnecessary pipeline executions and increased infrastructure costs.

Proposal

Implement a new "Optimize for MR approval policies" checkbox option that:

Automatically configures the policy to run scans in both source and target branches for open merge requests
Enforces the use of .latest templates to ensure MR pipelines are used correctly
Provides clear explanatory text about what this configuration does

Implementation Details

Update the ScanExecutionPolicyEditor.vue component to include the new strategy option
Add the necessary state management for this option
When checked:
- Set branch type to target_default in the backend configuration
- Ensure template selection is set to .latest
- Hide conflicting options to prevent confusion
Add appropriate tooltips and help text to explain the benefits
Implement behind a feature flag for controlled rollout

Technical Considerations

This feature depends on the backend implementation of target_default branch type
The UI should gracefully handle cases where the backend feature is not available
We should ensure the state is properly preserved when editing existing policies

Acceptance Criteria

New "Optimize for MR approval policies" checkbox is added to the SEP editor
When checked, it correctly configures the policy for MR approval compatibility
Clear explanatory text is shown to users about what this option does
The UI handles existing policies correctly when editing
Feature is implemented behind a feature flag
Documentation is updated to explain the new option