#145 Follow Up: Update the `ImageRegistry` source's `Unpack()` method asynchronous

Following operator-framework/catalogd#145, we should update the `Unpack()` method of `ImageRegistry` source to be asynchronous. This will ensure we don't create a reconciliation loop block on the actual pulling and layer unpacking of the catalog image.

**Acceptance Criteria**:
- The `Unpack()` method of the `ImageRegistry` source is asynchronous