Add possibility to include router via string

aiogram/utils/imports.py

@@ -0,0 +1,23 @@
+import importlib
+from typing import Any
+
+
+def import_module(target: str) -> Any:
+    if not isinstance(target, str):
+        raise ValueError(f"Target should be string not {type(target).__class__.__name__}")
+
+    module_name, _, attr_name = target.partition(":")
+    if not module_name or not attr_name:
+        raise ValueError(f'Import string "{target}" must be in format "<module>:<attribute>"')
+
+    try:
+        module = importlib.import_module(module_name)
+    except ImportError:
+        raise ValueError(f'Could not import module "{module_name}".')
+
+    try:
+        attribute = getattr(module, attr_name)
+    except AttributeError:
+        raise ValueError(f'Module "{module_name}" has no attribute "{attr_name}"')
+
+    return attribute