add support for partial scanning (#297)

Closes [SMS-545](https://linear.app/semgrep/issue/SMS-545). More context
in the ticket.

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

- [x] I ran `make setup && make` to update the generated code after
editing a `.atd` file (TODO: have a CI check)
- [x] 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

Author: amchiclet

semgrep_output_v1.atd

@@ -1873,3 +1873,13 @@ type function_return
   | RetResolveDependencies of (manifest * resolution_result) list
   | RetDumpRulePartitions of bool
 ]
+
+(* ----------------------------- *)
+(* Partial scans. Experimental and for internal use only. *)
+(* ----------------------------- *)
+
+type partial_scan_result
+    <python decorator="dataclass(frozen=True)"> = [
+  | PartialScanOk of (ci_scan_results * ci_scan_complete)
+  | PartialScanError of ci_scan_failure
+]