Allow opting-in for no credentials mount

[utikeev]

Already mentioned in this issue, but got eventually buried.

The problem is that mounting public buckets via gcsfuse requires some valid credentials present even though REST API doesn't. It would be great to have some opt-in flag like --allow-anonymous, which will prevent failing in case of nil tokenSrc, but rather create an anonymous connection (maybe with some warning).

[sethiay]

Thanks @utikeev for the request here. We will evaluate this request and update here accordingly. Till then, we suggest to use default credentials (e.g. set up via Google Cloud CLI) to access public buckets.

[marcoa6]

Supported with v2.1 through --allow-anonymous flag