Add support for service account impersonation

[andrewnicolalde]

This PR adds support for service account impersonation. It depends on the changes introduced in grafana/grafana-google-sdk-go#12

Note: I have not been able to get a binary built and written to disk so that I can test that these changes produce the desired result. If you have any advice as to how I can do so, that would be much appreciated!

TODO: Make changes to configuration web page to allow configuration via Grafana UI.

[CLAassistant]

All committers have signed the CLA.

[zoltanbedi]

Thanks for creating this PR @andrewnicolalde can you please sign the cla?

[andrewnicolalde]

@zoltanbedi I have now signed the CLA 😃.

[zoltanbedi]

@andrewnicolalde thanks. I'll take a look at your other PR first in the sdk repository.

[andrewnicolalde]

@zoltanbedi Now that we've added support for service account impersonation in grafana-google-sdk-go, what remains to be done to get a new version of this plugin released?

I know for certain that we need to add SA impersonation options to the configuration web pages for the plugin so that it can be configured through the Grafana UI.

[zoltanbedi]

The data source config needs a new option in https://github.com/grafana/grafana-google-sdk-react. I'm on it, going to open a PR soon. Once that is merged and released we can merge this.