Add advanced_sast_partial_scan input

What does this MR do and why?

Adds the advanced_sast_partial_scan input to the SAST component to support differential scanning for Advanced SAST.

This provides parity with the ADVANCED_SAST_PARTIAL_SCAN CI variable, allowing component users to enable differential scanning.

Closes issue Add advanced_sast_partial_scan input to sast co... (gitlab-org/gitlab#578067 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Start an Advanced SAST job configured with SAST components and set the advanced_sast_partial_scan input. See this example template

2. Observe in the Advanced SAST job that a differential scan ran by looking for this log entry:

   ADVANCED_SAST_PARTIAL_SCAN=differential

   • See this example job for reference