Manage .netrc files for workers not on the same filesystem

[gjoseph92]

Re #29: sounds like we currently need to have a .netrc file for libcurl to access NASA URS data: https://gist.github.com/scottyhq/c4a4e889b58a0a153dd5fb18bad9f3e8#gistcomment-3713665.

I would like to make the auth/env experience much smoother and automatically derive from the local environment in the future, but as a first step, let's just provide a path to make this work.

Ideas:

• Create a Netrc class that copies your local .netrc into memory, and when used as a contextmanager, writes the contents to a tempfile, and sets the path as the NETRC env var. Creating and deleting the tempfile per asset is probably a bad idea, though
• Create a worker plugin for copying the local .netrc to a tempfile on workers, and setting NETRC. Probably more reasonable for performance.