Support hot backups

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**Is your feature request related to a problem? Please describe.**
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lack of support to hot backups is preventing automate backup creation.
[1] doesn't have the type of backup variable causing every backup created using the client library to be default.

**Describe the solution you'd like**
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Support the `backup_type` variable as it works in gcloud[2] and the API[3]

**Describe alternatives you've considered**
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- Using as curl to call the API directly, however this needs to generate the token, and manage the HTTP request
- Using gcloud command, however this way doesn't scale to be automated.

**Additional context**
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[1]: https://github.com/googleapis/python-bigtable/blob/main/google/cloud/bigtable/backup.py
[2]: https://cloud.google.com/sdk/gcloud/reference/bigtable/backups/create#:~:text=progress%20to%20complete.-,%2D%2Dbackup%2Dtype,-%3DBACKUP_TYPE
[3]: https://cloud.google.com/bigtable/docs/reference/admin/rest/v2/projects.instances.clusters.backups#Backup:~:text=for%20the%20backup.-,backupType,-enum%20(BackupType