Create a backup performance baseline

[regevran]

When this issue is done we have a measurable, repeatable and consistent test for backing up a cluster using rclone.
These should be a group of tests, i.e. one configurable test with varies: nodes, data size.

[regevran]

This issue was originally created to compare rclone performance vs. Scylla Core performance when it comes to reads/writes during backup.
General question: What do you think, Is it worth doing?

Please note that #9403 is implemented and validates that Scylla Core does not interfere with reads/writes while it backups (this test compares reads with backup vs. read without backups, nothing to do with rclone).