build(package): add py.typed and include it in flit config

This adds a py.typed marker file to the google.adk package and updates
the Flit configuration in pyproject.toml to include it in the distribution.

This ensures the package is PEP 561 compliant and allows static type
checkers (like mypy and pyright) to recognize that the package supports type hints.

Author: mohamadghaffari

pyproject.toml

@@ -145,6 +145,7 @@ exclude = ['src/**/*.sh']
 
 [tool.flit.module]
 name = "google.adk"
+include = ["py.typed"]
 
 [tool.isort]
 profile = "google"