Adds new auth mode to use the default google auth client

[lmartelli]

Works out of the box when deployed on GCP, or using GOOGLE_APPLICATION_CREDENTIALS env var for local dev.

[lmartelli]

I think this makes AUTH_MODES.OAUTH redundant, so I removed it but let useOAuth2Client for backward compatibility.

[suciubogdan]

+1 for this to get merged if it actually works.

[horiken4]

👍

[horaklukas]

It would be fantastic to have this in the library 😍

[MacieGx]

+1

[theoephraim]

Thanks for contributing. I was in the middle of a big typescript rewrite that took longer than expected...

I worked this functionality into the new code - see https://theoephraim.github.io/node-google-spreadsheet/#/guides/authentication?id=adc