Provide SARIF to GL SAST report converter

Proposal

We do not currently provide a means by which a customer can convert a generic SARIF report format into the gitlab SAST report format.

The library we use internally is part of our report golang library, you can see an example of its usage in our semgrep analyzer.

This is primarily used internally, hence, not separately documented. It could be nice to have a self-contained converter but there isn't one at present. Alternatively, we could explore allowing uploads of SARIF reports directly and handling the conversion within our report parsing functionality.