publishergw: provide the revision to resource revision mappings created with `release` for charms

[Alex Lowe]

The release endpoint for charms allows mapping resource revisions to specific revisions, but we don't have a way to retrieve that data afterwards.

Ideally, list_resource_revisions would provide an array of workload revisions the resource revision maps to, and list_revisions would provide the resource revisions attached to that workload revision.