feature: Conda env extension hooks

[saikonen]

We need some additional extension capabilities to be able to implement #1796 as an extension, these being:

• add 'docker' to list of supported environments for @conda and @pypi
• ability to fallback to the core Conda environment initialization for a subset of steps only.

Note: the changes to conda_environment.py would at the moment result in any plugin relying on the new only_steps= to break with the bleeding edge nflx-extension conda implementation

[savingoyal]

Would hardcoding docker for now or relying on a property in docker_environment to ensure compatibility work better? Managing it through config seems a bit anti-pattern-ish for the long-term since it is not a user exposed config

[saikonen]

I thought on this for a bit and it felt a bit more natural to implement a way for the extensions to extend a supported list of environments, rather than them implementing a method for the decorator to call.

so the alternative approach here would be to implement something like the following, but only in environments that we support pypi/conda on?

# Environment
def _is_supported_deps_deco(self, deco_name):
    return deco_name in ["conda", "pypi"]

and in the decorators env validation

# Decorator
try:
    if not environment._is_supported_deps_deco(self.name):
        raise InvalidEnvironmentException("...")
except Exception:
    # env does not even implement the method for checking
    echo("environment does not support pypi/conda")
    raise

The main drawback with this approach is that we are now missing a list of environments that do support conda/pypi, so we can't display a helpful list to the user

[saikonen]

Let's go with the hardcoded option for the time being. Left a TODO regarding future improvements for extending the supported venv list.