Upstream Litestream GCS application credentials in config

[epompeii]

Current Litestream does not support taking in the GCS application credentials path as part of the config file: https://github.com/benbjohnson/litestream/blob/e4254bbf69893cf5e17c52df9036bfa35e6941ae/cmd/litestream/main.go#L538

It has to be passed in using the GOOGLE_APPLICATION_CREDENTIALS environment variable, which is... frustrating.
Upstream a fix adding it as key in the config file, like all the other schemes.