Support flatpaks

[ahopgood]

Describe the feature:
Support verifying installed flatpak packages with the goss add package command.

Describe the solution you'd like
Flatpak's info command could be queried to check if a flatpak is installed and what version:

flatpak info fr.handbrake.ghb

Produces something like this

HandBrake - Video Transcoder

    ID: fr.handbrake.ghb
  Ref: app/fr.handbrake.ghb/x86_64/stable
  Arch: x86_64
Branch: stable
Version: 1.10.1
License: GPL-2.0
Origin: flathub
Collection: org.flathub.Stable
Installation: system
Installed: 115.4 MB
  Runtime: org.gnome.Platform/x86_64/48
      Sdk: org.gnome.Sdk/x86_64/48

    Commit: aca0413f1b0300632d8913fe83f2a786902c452529aa564b4ceefe2901ef1c61
    Parent: 8b6836a05d156d444ba2aaebd5ed973b552e780fa705c19566e6a48b0c9beb32
  Subject: Update HandBrake to 1.10.1 (32049c2e9538)
      Date: 2025-08-25 14:46:39 +0000

Describe alternatives you've considered
Currently I'm using the info command with contain-substring to assert the flatpak installation.
Whilst workable I feel supporting via the goss add package fits with the goss project's ease of use aims.

command:
  flatpak info fr.handbrake.ghb:
    title: verify flatpak install of handbrake
    exit-status: 0
    stdout:
      and:
        - contain-substring: 'ID: fr.handbrake.ghb'
        - contain-substring:  'Version: 1.10.1'
    stderr: ""
    timeout: 10000