Shared GCS delegation tokens

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This change introduces new configuration variable applicable to GCS connector fs.gs.delegation.token.instantiation_strategy. It defines how delegation tokens are instantiated.

Its default value is 'GCS_TOKEN_INSTANTION_STRATEGY' and it does not change current functionality. Gcs tokens are instantiated per service and per renewer.

When it is set to 'SHARED' - GCS delegation tokens are shared across multiple storage buckets. Tokens are still unique per renewer.

This change applies to user authentication only and allows controlling user policies via Ranger plugin for jobs that use multiple GCS buckets at the same time.

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/gcbrun