Add the option to specify which format you want to output, defaulting to sarif.

Add the option to change the output of the report from the gitleaks scan.
I think it's useful to have SARIF as the default, as Github actions can upload this for code scanning.
please see PR #34 for a proposed solution.

Start by reviewing PR #34, which contains the proposed solution, and inspect how this GitHub Action currently receives and emits the Gitleaks report. Done means adding a configurable output format with SARIF as the default and confirming the action still produces the expected report.

Written by the indexing model from the issue text.