Do not save credentials in plaintext (at least, not as the default persistence mode)

[mfisher87]

Earthdata documentation may recommend using .netrc files to store credentials, but those files are still plaintext files. They do provide security advantages by mapping credentials to hostnames, but that doesn't negate the security concern of exposing the credentials.

Storing credentials in plaintext is especially problematic when we're training users (who are not security experts) to work on untrusted/shared machines/filesystems in the cloud and on JupyterHubs.

IMO, we should default to persisting only tokens!