[MR Widget] Allow fetching highlights

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Summary

The MR Widget currently uses the /security_reports?type=<report_type> REST endpoint to fetch the findings. We then iterate over the findings, and calculate the highlights. This is ineffective for two reasons:

1. It does not allow us to understand whether there are more findings or not (because the response is limited to 25 added and 25 fixed findings)
2. It's not effective to fetch all finding data to calculate the highlights

To overcome this, we need a way to fetch the highlights directly from the backend.

Implementation plan

• TBA