Add an optional field to the internal API to list target files (#430)

This is used by https://github.com/semgrep/semgrep-proprietary/pull/5274

- [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: mjambon

semgrep_output_v1.atd

@@ -2723,6 +2723,7 @@ type targeting_conf
   max_target_bytes : int;
   respect_gitignore : bool;
   respect_semgrepignore_files : bool;
+  ~extra_gitignore_patterns_to_exclude_git_untracked_files : string list;
   ?semgrepignore_filename : string option;
   always_select_explicit_targets : bool;
   explicit_targets