Make decorators pluggable

[dimberman]

This PR will allow users to add custom "@task.____" decorators by adding
to their setup.cfg files. This will make decorators seem native to
airflow while living in provider packages.

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[ashb]

General comments: I rather than airflow.decorator_implementations.python etc I would just leave them as airflow.decorators.python -- I don't think we really need to separate the folders

Have you played with creating a type-stub file for the attrs on the @task decorator? Does it work for IDE completion?

[dimberman]

@ashb unfortunately the problem with leaving them where they are is that because we are loading this stuff in the __init__ file, it leads to an infinite loop.

[ashb]

@ashb unfortunately the problem with leaving them where they are is that because we are loading this stuff in the __init__ file, it leads to an infinite loop.

Given python operator is built in, for the sake of efficiencey of parsing it might be worth not having an entrypoint for task.python etc. anyway.

[ashb]

Probably worth linking to https://setuptools.readthedocs.io/en/latest/userguide/entry_point.html

[dimberman]

@potiuk PTAL I think this addresses what you wanted?

Changes addressed

[dimberman]

@potiuk just realized I need to fix the documentation too. Doing that now.

[dimberman]

@potiuk Ok now it's updated PTAL thank you :)