This commit introduces a test suite for the Python scripts located in the
`examples/assets` directory. Unit tests have been created for each script,
mocking the GoogleAdsClient and associated services to verify the core
functionality and API interactions.

The tests cover:
- `add_call.py`
- `add_hotel_callout.py`
- `add_lead_form_asset.py`
- `add_prices.py`
- `add_sitelinks.py`
- `upload_image_asset.py`

Key aspects verified:
- Correct creation and configuration of various asset types.
- Proper linking of assets to accounts or campaigns.
- Appropriate parameters being passed to API service calls.

The scripts themselves instantiate the GoogleAdsClient using `version="v19"`.
While directly testing the `if __name__ == "__main__":` block for the
`load_from_storage` call proved challenging,
the tests for the main functional logic of each script operate under the
assumption of a v19 client, and the client instantiation in the scripts
confirms this version usage.

`__init__.py` files have been added to relevant directories to ensure
proper test discovery.

This commit addresses the following comments from PR #987:

- Modifies noxfile.py to ensure that tests in examples/assets/tests/ are discovered and executed.
- Removes the .gitkeep file from examples/assets/tests/ as it's no longer necessary due to the addition of test files.
- Adds the standard Apache 2.0 license header to all new test files in examples/assets/tests/.

This commit incorporates your feedback to:

1. Ensure all asset example scripts in `examples/assets/` and their corresponding tests in `examples/assets/tests/` are aligned with API version v20.
2. Update the copyright year to 2025 in the license headers for these files.

The `noxfile.py` was previously updated to discover and run tests from `examples/assets/tests/`.
The `.gitkeep` file in `examples/assets/tests/` was removed as it's no longer necessary.
License headers were added to all new test files in `examples/assets/tests/`.

All tests were run successfully after these changes.