feat(metrics): add findings_by_product field for CI scan stats payload

[zzeleznick]

Description

This PR supports adding the number of findings per product for our CI runs. Follow-on to #210.

Addresses: GROW-104

Test Plan

• I ran make setup && make to update the generated code after editing a .atd file (TODO: have a CI check)
• I made sure we're still backward compatible with old versions of the CLI.
  For example, the Semgrep backend need to still be able to consume data generated
  by Semgrep 1.17.0.
  See https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades

[github-actions]

Backwards compatibility summary:

Checking backward compatibility of semgrep_output_v1.atd against past version v1.29.0
Skipping v1.30.0 because commit 78720c795cd5a186f5102c87125ef876c6435a0c has already been checked
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.33.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.34.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.39.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.49.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.52.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.54.2
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.55.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.56.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.57.0
Skipping v1.58.0 because commit 4cc11b00d411c02fc611aa8c78a336520438fb48 has already been checked

[aryx]

would be better to use an actual enum ... We actually have a type product.
but yes maybe you can't use an enum for the key of a dictionnary with ATD.