Use "medusa status" to compute the Backup Summary

[adejanovski]

Currently the BackupSummary which gets written into the MedusaBackup status gets pulled from listing all the backups and filtering to get the one we're looking for.

We could instead add an operation in the GRPC server of Medusa to expose the equivalent of the medusa status command.

Definition of Done

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1. The status command is exposed in the grpc server
2. The status command in Medusa shows file count and sum size #1117
   done +1
   
3. k8ssandra-operator uses the new Status command from the grpc server to fill in the MedusaBackup CR status instead of listing all backups
4. Expose backup size in MedusaBackup CRD #1122
   done +1
   

Edit: Before even starting, ticking the first task as done because we already have the status exposed, plus we recently made it better in thelastpickle/cassandra-medusa#660.

[rzvoncek]

Completed all sub-tasks. Closing.