Align sm-agent and Scylla object storage configurations

[Michal-Leszczynski]

The description of this issue and possible solutions can be found in this doc.

The problem is that sm-agent configures object storage access via scylla-manager-agent.yaml, but Scylla uses object_storage.yaml for this purpose. Those files have different formats and it's not obvious how to match object storage configurations between them.

One of the possible solutions to this problem is #4161, but we haven't yet decided if that's the way to go.

UPDATE:
We decided that Scylla will expose an endpoint which would (issue):

• list object storage configurations
• tell which configuration is the default

Extending location flag with endpoint instead of provider is still needed (issue).
If user runs backup/restore just with the provider specified, SM will:

• use the default Scylla object storage configuration
• use the Rclone configuration for specified provider
• validate that both configurations match
  If user runs backup/restore with specified endpoint, SM will:
• use the provided endpoint Scylla object storage configuration
• use Rclone configuration for matching provider
• validate that both configurations match

Note that different nodes might have different configurations, so the validation needs to be per node (even though we allow for specifying provider/endpoint only per DC with --location falg)