* fixes #1712

* test: fix incorrect method call

* docs: add for validatePredicate

* docs: incorrect return value

* Fix broken GCS support that's throwing NPEs

The call to the `getHost()` method on the `URI` instance returns null
for a GCS uri like `gs://my_bucket/my_object`. Checking the URI docs
from Oracle
(https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/net/URI.html)
we should be using `getAuthority()`.

This commit changes the method call resolving the NPE. I've manually
tested with a test case hitting a publicly accessible object in Google
Cloud Storage (since no unit tests exist).

* fix typo in Google Cloud doc url

* Support GCP environmental service account auth

Previously, we forced using service account private keys set using the
GOOGLE_APPLICATION_CREDENTIALS environment variable pointing to a
generated json private key. This isn't required in native GCP
environments as the official Google Cloud clients will identify if
they're running in a GCP service (e.g. GAE, CloudRun, GCE, etc.) and
provide automatic authentication as a service account.

This change adds a new "authenticationType" and renames the old type:

- SERVICE becomes PRIVATE_KEY to explicitly describe that it's expecting
  to use a private key file.
- GCP_ENVIRONMENT is added to be used when running in GCP.

The logic is that for PRIVATE_KEY usage we still check the environment
for a set GOOGLE_APPLICATION_CREDENTIALS value. A runtime error is
thrown if it's not set.

For GCP_ENVIRONMENT, the environment check is ignored as the Google
client library will take care of validation.

* Update Google Cloud documentation for new auth

Add in details on the new authentication types and behavior, replacing
the old settings. Tweak some verbiage.