Add filestore index mutex

[modular-magician]

Fixes #9007

User experienced an internal error when creating two filestore instances in parallel. Adding an mutex at the project level to prevent multiple instances operating simultaneously.

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Release Note Template for Downstream PRs (will be copied)

filestore: fixed an issue in `google_filestore_instance` where creating two instances simultaneously resulted in an error.

Derived from GoogleCloudPlatform/magic-modules#4886

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