Add generic reporting ability to Task

Is your feature request related to a problem? Please describe.

There appears to be a general demand for tasks to report back task specific information for various use cases. Possible examples include:

• https://github.com/pulp/pulpcore/issues/3137
• https://github.com/pulp/pulp_rpm/issues/2722
• reporting estimated (maximum) disk size requirements

Describe the solution you'd like

The idea would be for the Task model to provide some relatively generic JSON field, that plugin authors can use to report back whatever information is relevant for some given plugin use case. The field might be called something like info or report.

For example, a plugin might want to implement a dry-run sync mode, that reports back a maximum disk size that the actual sync is expected to need. A task would be started that analyzes the upstream metadata for file sizes, and reports the estimate back to the new report/info field. Users can then access the information using whatever interface the plugin author defined, perhaps something like get <task_href> | jq '.info.estimated_max_disk_size'.

Start with the Task model and review how task data is exposed through the interface described in the issue. Compare the referenced use cases and decide the generic JSON field's scope, plugin contract, and access pattern. Done means plugin authors can report arbitrary task-specific information and users can retrieve it reliably.