Add plugins to get data from NASA, with examples

[reidsunderland]

A poll plugin to poll NASA's CMR API, and an after_accept plugin that uses a username + password to login to https://urs.earthdata.nasa.gov and generate/retrieve a bearer token for the account and add it to the credentials DB for every baseUrl processed by the plugin. The bearer tokens expire every 90 days, so it would be inconvenient to put the tokens in credentials.conf and update manually every 90 days.

[petersilva]

This looks great! The comments are really helpful/explanatory... I really like the idea of modifying the credentials db to include the bearer token... Probably something we can use elsewhere.

[reidsunderland]

Thanks :) I thought it was a good example that should be in the public repo.

I really wanted to avoid re-implementing HTTP downloading code to use cookies like I did for the v2 version, so I'm happy the bearer token worked