Improve dependency management

[ZanSara]

Improve dependecy management by introducing extras_require and migrating to setup.cfg and pyproject.toml.

Related to #1979

[ZanSara]

@tholor I have plenty of testing to do still, but this branch is already working on Python3.9 on Linux, so feel free to try it out and let me know if you encounter errors.

The dependency groups are also just stubs, there are a lot of dependencies that I'm not quite sure about, so if you already see something out of place let me know and I'll move them 🙂

[ZanSara]

Example of workflow for document stores:

$ pip install .

... pip logs ...

$ python
>>> import haystack
>>> from haystack.document_stores import FAISSDocumentStore
>>> e = FAISSDocumentStore()
Traceback (most recent call last):
  File "/home/sara/work/haystack/haystack/utils/import_utils.py", line 28, in safe_import
    module = importlib.import_module(import_path)
  File "/usr/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 850, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/home/sara/work/haystack/haystack/document_stores/faiss.py", line 13, in <module>
    import faiss
ModuleNotFoundError: No module named 'faiss'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sara/work/haystack/haystack/utils/import_utils.py", line 42, in __init__
    raise ImportError(f"Failed to import {classname}. "
ImportError: Failed to import FAISSDocumentStore. Run 'pip install farm-haystack[faiss] to install them.'
>>>
$ pip install .[faiss]

... pip logs ...

$ python
>>> import haystack
>>> from haystack.document_stores import FAISSDocumentStore
>>> e = FAISSDocumentStore()
>>>

For other optional modules like rest or ui, the process is more eager instead:

$ pip install .

... pip logs ...

$ python
>>> import rest_api.application
Traceback (most recent call last):
  File "/home/sara/work/haystack/rest_api/application.py", line 10, in <module>
    import uvicorn
ModuleNotFoundError: No module named 'uvicorn'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sara/work/haystack/rest_api/application.py", line 20, in <module>
    raise ImportError("Failed to import the REST API due to missing dependencies. Run 'pip install farm-haystack[rest]' to install them.") from ie
ImportError: Failed to import the REST API due to missing dependencies. Run 'pip install farm-haystack[rest]' to install them.
>>>
$ pip install .[rest]

... pip logs ...

$ python
>>> import rest_api.application
>>>

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