Adding 'setup.py' and 'pyproject.toml' for Local Installation

[d-kleine]

Is your feature request related to a problem? Please describe.
Currently, haystack and its related PyPI packages (e.g., haystack-core-integrations) lack setup.py and sometimes pyproject.toml files in their repositories. This creates challenges for developers who want to install the packages locally for testing or development purposes. Without these files:

• Local installations (e.g., via pip install -e .) are not straightforward.
• Developers cannot easily modify the source code and test changes locally without complex workarounds.
• The absence of a pyproject.toml file limits compatibility with modern Python packaging standards (PEP 517/518), making it harder to manage dependencies and build processes.

Describe the solution you'd like
I propose the addition of both setup.py and pyproject.toml files to all related repositories. This would provide a robust and flexible solution for local installation, testing, and development workflows.

Describe alternatives you've considered
None

Additional context
Adding these files aligns with Python packaging best practices and enhances compatibility with modern tools (e.g., Docker). In my opinion, this feature would significantly improve the developer experience and encourage more contributions from the community.