get used_size of a backup from simplyblock API

[boddumanohar]

partially fixes: simplyblock/vela#195

currently we size_bytes of a Backup from CSI VolumeSnapshot's RESTORESIZE field. But there is not field to get the used size. So as a part of this PR, we get the used size by calling Simplyblock's storage API

[mxsrc]

Generally this looks good, as usual I'm hesitant to write things to the database. This doesn't look like it's a high bandwidth thing so I'd prefer simply querying the API until we get issues with that.

Beyond generally that general thought I'm not sure about the semantics of used_size on snapshots, is that constant? If two snapshots contain a specific change and one of them get's deleted, wouldn't the used_size of the other increase?

[mxsrc]

Why do we allow for content_name to be None only to immediately raise if that's the case?

[boddumanohar]

the was done to deal with type checker.

    content_name = status.get("boundVolumeSnapshotContentName")
    size_bytes = quantity_to_bytes(status.get("restoreSize"))
    try:
        used_size_bytes = await _snapshot_used_size(content_name)

the content_name that is passed as input is possible to be both "str" and "None". That's why have allowed both and parameter.

happy to accept any alternative sugestions.

[mxsrc]

This sanitization should happen on the caller-side, this is not a failure of the function itself.

[boddumanohar]

The reason to store this in Database is:

This PR partially solves, simplyblock/vela#195

As a part of branch list API, we return used_resources. The goal is to add a new additional field snapshot_used_size (the field name is subject to change).

"used_resources": {
             "milli_vcpu": 0,
             "ram_bytes": 0,
             "nvme_bytes": 0,
             "iops": 0,
             "storage_bytes": null
+++      "snapshot_used_size": <sum of all used snapshot sizes of this branch>
    },

If the details are stored in database the computation of sum of all snapshots of a branch becomes easier and less costly.

[mxsrc]

According to the discussion with Michael we need to take more care when storing the used resources in the database. Bascially, at any point where snapshots are deleted, the values would have to be updated.