Base backups are single-threaded and ignore concurrency settings

The `plugin-barman-cloud` performs base backups using a single upload stream. Unlike WAL archiving (which supports `maxParallel`), there is no way to parallelize the data backup phase. This results in bottlenecks for large databases.

This option:
```
$ kubectl explain objectstore.spec.configuration.data.jobs
GROUP: barmancloud.cnpg.io
KIND: ObjectStore
VERSION: v1

FIELD: jobs

DESCRIPTION:
The number of parallel jobs to be used to upload the backup, defaults
to 2
```
is not applicable due to limitation (only for rsync):
https://github.com/EnterpriseDB/barman/blob/ff040cd1feee805da3e4662a187c583f8730dfab/docs/user_guide/configuration.rst?plain=1#L336

> parallel_jobs

> Controls the number of parallel workers used to copy files during backup or recovery.
It must be a positive integer. Default is ``1``.

> note::
> Applies only when ``backup_method = rsync``.

Start by tracing the plugin-barman-cloud base-backup upload path and compare it with WAL archiving's maxParallel handling. Determine how the data upload can honor a configurable parallel-job setting; done means the base backup phase supports parallel uploads without changing the existing default behavior.

Written by the indexing model from the issue text.