A number of commands in Airflow relies on the fact that
providers configuration is loaded. This is a rather fast operation
as it does not involve any importing of provider classes, just
discovering entrypoints, running them and parsing yaml configuration,
so it is a very low sub-second time to do it.

We cannot do it once in settings/config because we actually need
settings/config to be pre-initialized without providers in order
to be able to bootstrap airflow, therefore we need to run
it individually in each command that can be run with the
"airflow" entrypoint. Decorator seems to be best suited to do
the job:

* easy to apply and not easy to forget when you create another
  command and look at other commands
* nicely wraps around local ProvidersManager import

There are exceptions for the "version" and "providers lazy-loaded"
commands because they are NOT supposed to initialize configuration
of providers.