* and stop setting default values in python operator code, so defaults can be set at the server side.

This issue is just for the PR and questions from maintainers. 
setup.py
"""
    Replaces extra requirement with provider package. The intention here is that when
    the provider is added as dependency of extra, there is no need to add the dependencies
    separately. This is not needed and even harmful, because in case of future versions of
    the provider, the requirements might change, so hard-coding requirements from the version
    that was available at the release time might cause dependency conflicts in the future.
    Say for example that you have salesforce provider with those deps:
    { 'salesforce': ['simple-salesforce>=1.0.0', 'tableauserverclient'] }
    Initially ['salesforce'] extra has those requirements and it works like that when you install
    it when INSTALL_PROVIDERS_FROM_SOURCES is set to `true` (during the development). However, when
    the production installation is used, The dependencies are changed:
    { 'salesforce': ['apache-airflow-providers-salesforce'] }
    And then, 'apache-airflow-providers-salesforce' package has those 'install_requires' dependencies:
            ['simple-salesforce>=1.0.0', 'tableauserverclient']
    So transitively 'salesforce' extra has all the requirements it needs and in case the provider
    changes it's dependencies, they will transitively change as well.
    In the constraint mechanism we save both - provider versions and it's dependencies
    version, which means that installation using constraints is repeatable.
    :param extra: Name of the extra to add providers to
    :param providers: list of provider ids
    """
    EXTRAS_REQUIREMENTS[extra] = [
        get_provider_package_from_package_id(package_name) for package_name in providers
    ]
def add_provider_packages_to_extra_requirements(extra: str, providers: List[str]) -> None: