Feature Request: List Outdated Packages

[aadrian]

• My proposal does not exist in the PLANNED & NOT PLANNED.md document
• I have not found any other opened issues on the same feature request
• I believe this fits within Warehouse's scope of Flatpak management

Describe the feature idea in detail

List Outdated Package(s).

Very often it's important just to see that a package(s) are outdated, and not update all of them, or the user is interested in updating only one - but it needs to know this, instead of blindly updating (like it happens with the flatpak update command).