Add peotry.lock file for pyproject.toml

After discussion with Kristian Reed (Snyk), he mentioned 2 different ways to list our requirements in the project:
1. to create a requirements.txt file
2. to have a pyproject.toml + poetry.lock files

Since a pyproject.toml file already exists, a poetry.lock should be added. It will help Snyk detect future vulnerabilities in the repository.

More information on poetry.lock files:
[https://python-poetry.org/docs/basic-usage/]