• Add GCP authentication support

What does this PR do?

• Enables ability to authentication using authn-gcp

What ticket does this PR close?

Resolves #222

Checklists

Change log

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR, and/or there is a follow-on issue to update docs, or
• This PR does not require updating any documentation

Manual tests

If you are preparing for a release, have you run the following manual tests to verify existing functionality continues to function as expected?

• Manually run Kubernetes-Conjur demo with a local authn-k8s client image build