Add additional support for TENSORSTORE_GCS_BACKEND environment variable

[copybara-service[bot]]

Add additional support for TENSORSTORE_GCS_BACKEND environment variable

This allows the user to configure the Tensorstore GCS backend (i.e. with gcs for http or gcs_grpc for grpc) for additional places in the code that did not previously have support, including when using ocdbt.

This also refactors some previously duplicated code to parse the gcs bucket/path and generate the kvstore spec into the existing gcs_utils.py file.

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