add support for partial scanning

[amchiclet]

Closes SMS-545. More context in the ticket.

This PR doesn't break semgrep's build and can be merged safely.

• 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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Skipping v1.31.1 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
Skipping v1.33.1 because commit 8849e56ddb0977e38a120a6cfbd1c396eb6fa15e has already been checked
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.34.0
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Skipping v1.38.1 because commit fd294683e7369cabf63738febeaba8a22c925187 has already been checked
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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
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.61.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.66.1
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.67.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.70.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.73.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.84.0
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Checking backward compatibility of semgrep_output_v1.atd against past version v1.90.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.91.0

[amchiclet]

If this gets approved this week, I'll wait for the release before merging.