Add an engine flavor type to report findings that can only come from pro (#226)

- [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: emjin

semgrep_output_v1.atd

@@ -165,11 +165,44 @@ type error_severity
   | Info <json name="info">
 ]
 
-(* TODO? PRO can actually have "flavors" (see Engine_type.ml) *)
+(* Used for a best-effort report to users about what findings they get with
+   the pro engine that they couldn't with the oss engine.
+
+   Interproc_taint = requires interprocedural taint
+   Interfile_taint = requires interfile taint
+   Other_pro_feature = requires some non-taint pro feature *)
+type pro_feature
+  <ocaml attr="deriving show">
+  <python decorator="dataclass(frozen=True)"> = {
+    interproc_taint: bool;
+    interfile_taint: bool;
+    proprietary_language: bool;
+}
+
+(* Report the engine used to detect each finding. Additionally, if we are able to
+   infer that the finding could only be detected using the pro engine, report that
+   the pro engine is required and include basic information about which feature is
+   required.
+
+   OSS = ran with OSS
+   PRO = ran with PRO, but we didn't infer that OSS couldn't have found this finding
+   PRO_REQUIRED = ran with PRO and requires a PRO feature (see pro_feature_used)
+   
+   Note: OSS and PRO could have clearer names, but for backwards compatibility
+   we're leaving them as is *)
+type engine_of_finding
+     <ocaml attr="deriving show">
+     <python decorator="dataclass(frozen=True)"> = [
+  | OSS 
+  | PRO
+   (* Semgrep 1.64.0 or later *)
+  | PRO_REQUIRED of pro_feature
+]
+
 type engine_kind
      <ocaml attr="deriving show">
      <python decorator="dataclass(frozen=True)"> = [
-  | OSS
+  | OSS 
   | PRO
 ]
 
@@ -233,7 +266,7 @@ type core_match_extra <python decorator="dataclass(frozen=True)"> = {
   ?fix: string option;
 
   ?dataflow_trace: match_dataflow_trace option;
-  engine_kind: engine_kind;
+  engine_kind: engine_of_finding;
   (* for nosemgrep *)
   is_ignored: bool;
   (* If present indicates the status of postprocessor validation. This field
@@ -298,7 +331,7 @@ type cli_match_extra = {
    * later on. *)
   ?dataflow_trace: match_dataflow_trace option;
 
-  ?engine_kind: engine_kind option;
+  ?engine_kind: engine_of_finding option;
 
   (* EXPERIMENTAL: If present indicates the status of postprocessor validation. This
    * field not being present should be equivalent to No_validator.