Add helper-style callback functionality

[solvingj]

The initial naive implementation is to return the credentials via stdout. This is inherently insecure and problematic, but still an improvement to storing credentials as environment variables. We can make minor some improvements on this approach, but it will never be secure enough for some cases.

This ticket is for the eventual addition of a generic cross-platform credential helper like this, which reads the credentials in from the keychain, and holds them in memory, and provides a more robust pattern for interacting with various credentials.