Native App CLI Login package

[rpwagner]

To make building CLI tools simpler, take the Native App login functionality and break it out into its own package, rather than copying from the Globus CLI or incorporating it into the SDK. The package should do something reasonable for token handling, like putting tokens under a $HOME/.globus-native-app/<app UUID>/ directory. Watch for collisions with other Globus packages that put files in the home directory. The package will go into PyPI. The documentation for the Native App CLI example will then use this package. This code will be in a separate repo.

Test by splitting out this from the Globus CLI.