Include sast reports endpoint in diffs-tab-pane data

What does this MR do and why?

This MR adds the /security_reports?type=sast endpoint path to diffs_tab_pane_data to be used by frontend in !119062 (closed).

The code is mostly taken from !118491 (closed) with few adjustments (e.g. feature flag name) because we came to the conclusion reusing /security_reports?type=sast would be enough to display SAST findings inline inside the diff tab instead of introducing a new endpoint to compare SAST findings for a specific merge request.

Resolves #389867 (closed).

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