Improve GcsPinotFs to Read Credentials Directly from User Configurations

Now, the [GcsPinotFs](https://github.com/apache/pinot/blob/master/pinot-plugins/pinot-file-system/pinot-gcs/src/main/java/org/apache/pinot/plugin/filesystem/GcsPinotFS.java) can only [read credentials from a file] (https://github.com/apache/pinot/blob/master/pinot-plugins/pinot-file-system/pinot-gcs/src/main/java/org/apache/pinot/plugin/filesystem/GcsPinotFS.java#L78) specified in the user configuration.

This needs to be improved for the following reasons:
1. Users need to create a file first, then provide this file name in the configuration, which is not so convenient.
2. If users don't have access to the container running pinot, then users cannot provide credentials.

Ideally, GcsPinotFs should be similar to [S3PinotFS](https://github.com/apache/pinot/blob/master/pinot-plugins/pinot-file-system/pinot-s3/src/main/java/org/apache/pinot/plugin/filesystem/S3PinotFS.java), allowing users to provide credential directly in configuration.