Add --cog-path CLI argument

This argument takes a list of strings (can be used multiple times for more than one value), validates that the path provided exists, and adds the path via `self._cog_mgr.add_path()`, so that cogs can be loaded from that path.

Closes Cog-Creators#6506

redbot/core/_cli.py

@@ -219,6 +219,15 @@ def parse_cli_flags(args):
         action="extend",
         help="Force unloading specified cogs.",
     )
+    parser.add_argument(
+        "--cog-path",
+        type=str,
+        default=[],
+        nargs="+",
+        action="extend",
+        help="Add a specific path to the list of cog paths. "
+        "This can be used multiple times to add multiple paths.",
+    )
     parser.add_argument(
         "--dry-run",
         action="store_true",

redbot/core/bot.py

@@ -1221,6 +1221,15 @@ async def _pre_connect(self) -> None:
                 )
             )
 
+        if self._cli_flags.cog_path:
+            for path in self._cli_flags.cog_path:
+                path = Path(path)
+                if not path.exists():
+                    log.warning("Cog path '%s' does not exist, skipping", path)
+                    continue
+                await self._cog_mgr.add_path(path)
+                log.info("Added cog path: %s", path)
+
         if packages:
             # Load permissions first, for security reasons
             try: