Add examples for BackupService and RestoreService

Proposal:
Add examples demonstrating how to use the BackupService and RestoreService as they are both currently undocumented.

Current behavior:
Examples are missing apart from brief comments in the source code.

Desired behavior:
It would be nice to have examples demonstrating how to use both services, possibly in the examples directory or in the README. Personally, I would like to use BackupService and RestoreService to perform migrations but am unsure how to use these services.

Alternatives considered:
Other alternatives to the BackupService and RestoreService is using the Influx CLI with Python subprocess calls, but this is less testable and not preferred.

Use case:
Performing backups or restores using the InfluxDB Python client would be much easier with documentation.