Allow use of the Instance Metadata Service instead of using an API Key

Hello,

I realized when browsing this project that this will require an API Key to work whereas the Webrisk API seems to be compatible with tokens generated by the IMDS service.
More specifically, using the library https://godoc.org/cloud.google.com/go/webrisk/apiv1beta1 that is referenced in the Webrisk doc doesn't require an API Key.

Is this a feature that is planned for this project?

Or would you be willing to accept a PR to add a new API struct that would implement the `api` interface and leverage the webrisk library above to use the IMDS instead of an API Key?