Feature request: Pinning resource revision numbers when exporting charms

[Lucas Gameiro Borges]

Hello!

An important feature that, as far as I'm aware, is currently missing from store-admin is the ability to pin specific resource revisions associated with charms. An example of this would be the way juju bundles work right now:

```
applications:
  postgresql-k8s:
    channel: 14/edge
    charm: postgresql-k8s
    constraints: arch=amd64
    revision: 123
    resources:
      postgresql-image: 321
```

It seems that currently the `resources.<resource_name>` field is being used as an override to local registry storage path. This may lead to confusion regarding possible pinning of resources for users coming from juju bundle syntax.

Let me know if any more context is needed here. Thank you!