Add custom update to update metadata in a separate call

[modular-magician]

This pull request make a custom 'updateIndex' method. Since if index.metadata need to update, not other index fields can be also updated as part of the same update call. So we need to make a separate update call for index.metadata.

index_update: fixed pre-update use unsupported nested updatemask "metadata.contentsDeltaUri". Use first layer update mask and update "metadata" field in a separate call.

Derived from GoogleCloudPlatform/magic-modules#11081

[github-actions]

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