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

[JulesDT]

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?

[rvilgalys]

Hello,

Sorry we're just getting to this now, I noticed this when a similar feature was requested in #52.

We've recently brought this repo into Google's internal source control & maintenance which now releases to Github, so cannot accept PRs right now -- however adding more than just API key auth is something I'd like to get added soon as it's been brought up with other customers. I will update here with details as they're available.

Thanks!