docs: add type hints to examples (things.py, things_asgi.py, things_advanced_asgi.py)

[Midhun1618]

Summary of Changes

Added type annotations to three example files as part of #1820:

• examples/things.py — added from falcon import Request, Response and typed the on_get responder
• examples/things_asgi.py — added from falcon.asgi import Request, Response and typed the on_get responder
• examples/things_advanced_asgi.py — added from falcon.asgi import Request, Response and from typing import Any, typed all responder methods, middleware process methods, hook functions, and helper methods throughout

Related Issues

#1820

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• Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
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  • Added docstrings for any new classes, functions, or modules.
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