Plugin registration update: Update it to use the new way of registration(through app)

Update this to use the new way of registration(through app):

`async def main():
"""Main entry point for the agent."""
prompt = 'hello world'
runner = InMemoryRunner(
agent=root_agent,
app_name='test_app_with_plugin',

# Add your plugin here. You can add multiple plugins.
plugins=[CountInvocationPlugin()],
)

# The rest is the same as starting a regular ADK runner.
session = await runner.session_service.create_session(
user_id='user',
app_name='test_app_with_plugin',
)
`

And maybe a few other places that might use the old way.