Auto-update repository access tokens

Support hands-free access to repositories that authenticate with short-lived access tokens.  When needed, call out to a configured shell script to update a configured system property with the repo's temporary password.

Implementation ideas: Receive from the shell script's stdout the content of a Java properties file.  Use a TTL property received from the shell script to set an expiration property – call the shell script only when the expiration property doesn't exist or will soon pass.  For security, prepend a configured prefix to the name of each system property to be set.

Document the configuration parameters.