fix #5622: backup client: properly handle rate/burst parameters

fix #5622: backup client: properly handle rate/burst parameters

The rate and burst parameters are integers, so the mapping from value
with `.as_str()` will always return `None` effectively never
applying any rate limit at all.

Fix it by turning them into a HumanByte instead of an integer.

To not crowd the parameter section so much, create a
ClientRateLimitConfig struct that gets flattened into the parameter list
of the backup client.

To adapt the description of the parameters, add new schemas that copy
the `HumanByte` schema but change the description.

With this, the rate limit actually works, and there is no lower limit
any more.

The old TRAFFIC_CONTROL_RATE/BURST_SCHEMAs can be deleted since the
client was the only user of them.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>