[CDTOOL-1717] Add support for the GCS Logging public-key flag

[rcaril]

Change summary

This PR adds support for the GCS Logging public-key flag.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

Are there any considerations that need to be addressed for release?

This PR is pending on the release of fastly/go-fastly#881, tests will fail accordingly prior.

[rcaril]

Confirmed with local testing that the flag correctly updates and retrieves a Public Key for a given GCS endpoint.

[kpfleming]

I'm guessing this needs a go-fastly release before the linter checks will succeed, right?

[rcaril]

I'm guessing this needs a go-fastly release before the linter checks will succeed, right?

Yup, exactly.