[Feat.] RDS: PITR for existing rds instance

[artem-lifshits]

Summary of the Pull Request

• Updated restore_point argument of opentelekomcloud_rds_instance_v3 resource allowing to
  restore RDS instance backup to existing RDS instance
• Parameter restore_from_backup is deprecated in favor of restore_point for resource/opentelekomcloud_rds_instance_v3

PR Checklist

• Refers to: [RDS]: implement PITR for existing opentelekomcloud_rds_instance_v3  #2776
• Tests added/passed.
• Documentation updated.
• Schema updated.
• Release notes added.

Acceptance Steps Performed

=== RUN   TestAccRdsInstanceV3RestoreToPITR_NewInstance
--- PASS: TestAccRdsInstanceV3RestoreToPITR_NewInstance (950.46s)
PASS

Process finished with the exit code 0

[otc-zuul]

Build succeeded.
https://zuul.otc-service.com/t/eco/buildset/8421650e99a34d6da24d133817b6b070

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