Add optional/required option for btrbk

Hi,

if some of the targets is not available, btrbk skips pruning local snapshots for security. In my concept, not all targets are always available at the same time, so I noticed local snapshots got never pruned. Please add an optional or required option for targets and pruning only takes the required targets into account.

Start by tracing how btrbk determines target availability and where local snapshot pruning is skipped when a target is unavailable. Define how optional and required targets should be represented and ensure pruning considers only required targets, while preserving the security behavior for required targets. Done means the new option is documented and pruning works when optional targets are offline.

Written by the indexing model from the issue text.