Firestore: Add support for Point-in-time-recovery (PITR)

[IchordeDionysos]

Adds support for Firestore Point-in-time-recovery (PITR).

Fixes hashicorp/terraform-provider-google#15788

Release Note Template for Downstream PRs (will be copied)

firestore: added `point_in_time_recovery_enablement`, `version_retention_period`, and `earliest_version_time` fields to `google_firestore_database ` resource

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[rileykarson]

Note: I need to deduplicate w/ #8863

[IchordeDionysos]

Ahh okay, I have search for public GitHub issues and did not find anything that's why I filed the issue and started working on it.
But nice to see that there is already a PR and it's already merged.

I don't think this PR is then needed anymore as it does not add anything new...

[rileykarson]

Closing as this looks redundant after that change!