Add version command, unit tests

[amrit110]

This pull request introduces several improvements to the testing infrastructure and command-line interface for the aieng_platform_onboard package. The main changes include adding a GitHub Actions workflow for running unit tests, enhancing the CLI with a version flag, and providing comprehensive pytest fixtures to support testing.

Testing infrastructure improvements:

• Added a new GitHub Actions workflow (.github/workflows/unit_tests.yml) to automatically run unit tests, collect coverage, and upload results to Codecov on pushes and pull requests.
• Added a tests/conftest.py file with a comprehensive set of pytest fixtures for mocking Firestore, HTTP requests, Google authentication, subprocesses, and console output, as well as providing sample data for participants, teams, and global keys.
• Added a module docstring to tests/__init__.py for clarity.

CLI enhancements:

• Added a get_version function and a --version command-line argument to the CLI, allowing users to display the installed package version. [1] [2] [3]

[codecov]

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