feature: allow rebuilding conda/pypi environments

[saikonen]

this PR adds a command for forcing a rebuild of the conda/pypi environment(s) of a flow.

# rebuild all environments
HelloConda.py --environment conda environment rebuild

# specify any number of steps to rebuild only their environments
HelloConda.py --environment conda environment rebuild --step start --step end

[romain-intel]

General comment: could we align on the names. The extension already does this: https://github.com/Netflix/metaflow-nflx-extensions/blob/main/metaflow_extensions/netflix_ext/plugins/environment_cli.py and for the limited functionality supported here, we could do the same names (ie: resolve --force) which may make things a bit more consistent.

[romain-intel]

No comment on the functionality or how you want to make it work but apart from a minor nit, the command looks fine.

[romain-intel]

nit: the other plugin uses those as argument resolve start end mid isntead of --step start --start end.