Automatically refresh credentials

[jlewi]

Automatically refresh at proto credentials.

The implementation isn't the most elegant. It would be nicer to do this in a more transparent way; e.g. using a RoundTripper or following the pattern of OAuth libraries that automatically refresh the credential everytime you call Token if necessary.

The current implementation uses our existing xrpc manager and fetches a new client with refreshed credentials if necessary.

• Also add a command to dump lists to a file
• So we can verify that a list is up to date.