Port taskgraph action from gecko_taskgraph

The documentation already says `You can generate actions.json locally by
running taskgraph actions.` however that command was never ported over
from gecko's `./mach taskgraph actions`

Author: Eijebong

src/taskgraph/main.py

@@ -710,6 +710,44 @@ def decision(options):
     taskgraph_decision(options)
 
 
+@command("actions", help="Print the rendered actions.json")
+@argument(
+    "--root",
+    "-r",
+    help="root of the taskgraph definition relative to topsrcdir",
+    default="taskcluster",
+)
+@argument(
+    "--verbose",
+    "-v",
+    action="store_true",
+    help="include debug-level logging output",
+)
+@argument(
+    "--parameters",
+    "-p",
+    default="",
+    help="parameters file (.yml or .json; see `taskcluster/docs/parameters.rst`)`",
+)
+def actions(args):
+    from taskgraph.actions import render_actions_json
+    from taskgraph.generator import TaskGraphGenerator
+    from taskgraph.parameters import parameters_loader
+
+    if args.pop("verbose", False):
+        logging.root.setLevel(logging.DEBUG)
+
+    try:
+        parameters = parameters_loader(args["parameters"])
+        tgg = TaskGraphGenerator(root_dir=args.get("root"), parameters=parameters)
+
+        actions = render_actions_json(tgg.parameters, tgg.graph_config, "DECISION-TASK")
+        print(json.dumps(actions, sort_keys=True, indent=2, separators=(",", ": ")))
+    except Exception:
+        traceback.print_exc()
+        sys.exit(1)
+
+
 @command("action-callback", description="Run action callback used by action tasks")
 @argument(
     "--root",