Create a new plugin hook for Upgrading packages

[fdosani]

So currently the packages are always upgraded via pip here.

Proposal is to make this a hook, something like:

@hookspec(firstresult=True)
def run_update(python_path: str, upgrade: List):
    """Update packages from upgrade list.

    Parameters
    ----------
    python_path : str
        The path to the python executable.
    upgrade : list
        The list of packages to upgrade
   
    Raises
    ------
    RuntimeError
        Error raised if the packages cannot be updated.
    """

This directly links back to a request in the conda plugin: capitalone/edgetest-conda#30
Enabling this functionality will enable the plugin to provide a spec to upgrade via conda.