Adopt optional dependencies

Having a community repository is great for collecting options for storage, etc. The current pyproject.toml forces installation of dependencies for all existing community extras whether they are desired or not.

It would be easier to use with a pip install google-adk-community[extra-name] syntax to allow for choice of redis, open memory, etc components.

Start by reading pyproject.toml and locating how the existing community extras and their dependencies are declared. Confirm the available component names, then make the packaging behavior support selecting components with the documented extra-name syntax instead of installing every community dependency; verify that the requested Redis and Open Memory choices can be installed independently.

Written by the indexing model from the issue text.