Forbid usage of *args and **kwargs in any node's __init__

[ZanSara]

Problem:

• The YAML validation code seems to ignore **kwargs.
• If a subclass uses **kwargs in its __init__ to transmit any unknown parameter to its superclass __init__, the validation for that node is going to fail and the nade cannot be instantiated, as in many cases the content of **kwargs is critical for the superclass.

Solution

• The YAML schema generator should look for **kwargs in the signature and, if detected, merge the superclass' schema with the subclass'.

@TuanaCelik

[ZanSara]

After a better inspection of the code, I realized that in fact we can't handle **kwargs properly. Once they go in the __init__() of the subclass we've got no guarantees that they are not meant to be used locally at least in part: assuming they will all be sent to the superclass is a rather implicit assumption that many custom (and future) nodes might not respect.

Given that **kwargs are generally not a necessity but simply a compactness measure, I propose to remove the usage of **kwargs from our nodes and document stores and forbid any future usage.

This will mean modifying:

• FARMReader Remove **kwargs from FARMReader #2413
• All the docstores that inherit from ElasticsearchDocumentStore, SQLDocumentStore and any other docstore using **kwargs Remove **kwargs from docstores' __init__ functions #2407

In the docstore cases, it will be enough to copy-paste the parent signature down to the subclasses.
For FARMReader, this will imply modifying some more internal classes to stop forwarding kwargs around when it's not needed. I will open separate PRs for these changes and merge them into this one.

[bogdankostic]

LGTM! To make the failing code-and-docs-check pass, I think you need to apply black manually to haystack/nodes/_json_schema.py and test/test_pipeline_yaml.py.

[ZanSara]

It sometimes fails when it checks too early. Given that the Code Style action passed, I think it's safe to merge. Thanks! 👍