feat(mal-deps): add flag to selectively enable mal-deps per scan (#434)

Right now mal-deps is either fully enabled or fully disabled at a
deployment level. This is problematic for testing improvements to
mal-deps on a per customer / project basis. This flag will let the CLI
opt in to mal-deps for a specific scan, even if it is turned off at the
deployment level.

- [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 1.50.0.
See
https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades
	  Note that the types related to the semgrep-core JSON output or the
	  semgrep-core RPC do not need to be backward compatible!
- [x] Any accompanying changes in `semgrep-proprietary` are approved and
ready to merge once this PR is merged

Author: nhakmiller

semgrep_output_v1.atd

@@ -1908,6 +1908,11 @@ type scan_metadata
   : string option;
   ~ecosystems: string list;
   ~packages: string list;
+  ?enable_mal_deps
+    <doc text="
+    Override to enable malicious dependency rules for this scan,
+    even if disabled at the deployment level.">
+  : bool option;
 }
 
 type ci_config_from_repo