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.