Add support for GCP Federation

[salrashid123]

Authenticating to GCP now supports various types of federation from AWS, Azure and generic oidc providers.

Basically, a customer can run this fluentd plugin in any of those enviornments without having to download and distriubte a service account credential file...i.,e they can just use the ambient credentisl they have already.

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this specific feature is detailed here:

• https://cloud.google.com/iam/docs/access-resources-aws#generate
• https://cloud.google.com/iam/docs/access-resources-oidc#generate

i've got a couple of live samples here for aws and generic oidc

happy to help with the gcp integration side but i don't really know ruby