Extend --location to accept raw endpoint instead of just the provider

[Michal-Leszczynski]

Right now the --location flag allows for specifying just the provider name, while the actual endpoint is always resolved on agent side:

  -L, --location [<dc>:]<provider>:<bucket>   A list of backup locations separated by a comma, specifies where to place the backup, the format is [<dc>:]<provider>:<bucket>.
                                              The '<dc>' parameter is optional it allows to specify location for a datacenter in a multi-dc setting, it must match Scylla nodes datacenter.
                                              The supported storage '<provider>'s are 'azure', 'gcs', 's3'.
                                              The 'bucket' parameter is a bucket name, it must be an alphanumeric string and **may contain a dash and or a dot, but other characters are forbidden**.

The resolving might consist of just checking hard-coded endpoint value in the scylla-manager-agent.yaml file (just for s3), or relying on rclone/SDKs to find the default endpoint for the machine (for all providers). In some cases this might not be flexible enough.

The Scylla object storage config identifies object storage by the endpoint. This means that we need to resolve provider into the endpoint when making a backup/restore with SM while using Scylla backup/restore API.
We can implement resolver simulating rclone/SDKs logic, but we shouldn't make it the only option, as it might not cover all the use cases.