Add initial project structure and tests for Elytra PIM Client

- Created pyproject.toml for project metadata and dependencies.
- Added requirements.txt for development and production dependencies.
- Implemented basic test structure in the tests module.
- Developed unit tests for ElytraClient, including Pydantic validation and error handling.

requirements.txt

@@ -0,0 +1,11 @@
+requests>=2.28.0
+python-dotenv>=0.21.0
+pydantic>=2.0.0
+
+# Development dependencies
+pytest>=7.0.0
+pytest-cov>=4.0.0
+black>=23.0.0
+isort>=5.12.0
+flake8>=6.0.0
+mypy>=1.0.0