[hack] Add structured json to hh --json

Summary: Referred to diffusion/FBS/browse/master/fbcode/hphp/hack/src/server/serverArgs.ml;ef20a30fdb088ec460ec4dfb744d1f9d2af89da6$100-107 as the basis for this change.

Reviewed By: 2BitSalute

Differential Revision: D15566968

fbshipit-source-id: 728c780be9deec24d01bea2145f570f6ce7a7819

Author: Young Kim

hack/utils/build_id.ml

@@ -40,3 +40,11 @@ let is_build_optimized =
  * v5 (hvvm 3.23, 17 Nov 2017) - 'hh_client lsp' stable
  *)
 let build_api_version = 5
+
+let build_version_json =
+  let open Hh_json in
+  JSON_Object [
+    "commit", JSON_String build_revision;
+    "commit_time", int_ build_commit_time;
+    "api_version", int_ build_api_version;
+  ]

hack/utils/build_id.mli

@@ -27,3 +27,4 @@ val build_minor_version : int
 val build_api_version : int
 val build_mode : string
 val is_build_optimized : bool
+val build_version_json : Hh_json.json