Add protocol field for filestore Instance product

[modular-magician]

Fixes hashicorp/terraform-provider-google#17523. As NFSv4.1 is in preview, this feature is available only for google-beta provider.

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
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Release Note Template for Downstream PRs (will be copied)

filestore: Added `protocol` property for `google_filestore_instance` instance to support NFSv3 and NFSv4.1

Derived from GoogleCloudPlatform/magic-modules#10324