AlloyDB BackupDR integration support

[varadvjoshi99]

Add support for restoring AlloyDB clusters via BackupDR backups and PITR via BackupDR data sources.

To restore a cluster directly from a backup, add the following line to your AlloyDB cluster:

restore_backupdr_backup_source {
backup =
}
To restore a cluster via PITR, add the following lines to your AlloyDB cluster:

restore_backupdr_pitr_source {
data_source =
point_in_time = <timestamp in RFC 3339 format>
}
Part of hashicorp/terraform-provider-google#13757. I am not fully closing the issue because I did not yet add support for data calls for backups as requested.

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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).

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alloydb: added `restoreBackupdrBackupSource`, `restoreBackupdrPitrSource`, and `backupdrBackupSource` fields to `google_alloydb_cluster` resource

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[varadvjoshi99]

Hi @rileykarson, please take a look, Thanks!