Command bot GHA v2 - /cmd <cmd> (paritytech#5457)

Closes paritytech/product-engineering#93

- Deprecates old command bot bash scripts. New cmd implementation is
re-written on Python
- Deprecates `sync` command, which was never used
- Benchmarks:
- Uses new
[frame-omni-bencher](https://crates.io/crates/frame-omni-bencher)
- Simplifies usage to only providing a runtime and/or pallet name (even
multiple runtimes or pallets)
- Supports sub-modules (like `/cmd bench --runtime dev --pallet
pallet_asset_conversion_ops`)
- Can regenerate all weights with one command (substrate, polkadot,
cumulus) for provided pallet(s) name
- Adds [subweight](https://crates.io/crates/subweight-core) diff as a
result of bench command

.github/scripts/cmd/_help.py

@@ -0,0 +1,26 @@
+import argparse
+
+"""
+
+Custom help action for argparse, it prints the help message for the main parser and all subparsers.
+
+"""
+
+
+class _HelpAction(argparse._HelpAction):
+    def __call__(self, parser, namespace, values, option_string=None):
+        parser.print_help()
+
+        # retrieve subparsers from parser
+        subparsers_actions = [
+            action for action in parser._actions
+            if isinstance(action, argparse._SubParsersAction)]
+        # there will probably only be one subparser_action,
+        # but better save than sorry
+        for subparsers_action in subparsers_actions:
+            # get all subparsers and print help
+            for choice, subparser in subparsers_action.choices.items():
+                print("\n### Command '{}'".format(choice))
+                print(subparser.format_help())
+
+        parser.exit()