Allow communication via the secret-service portal

[Cogitri]

Right now apps using secret-service have to punch a hole through the Flatpak sandbox to allow communication with org.freedesktop.secrets . Secret-service should provide the capability to use the secrets portal so this isn't necessary anymore.

ashpd's SecretProxy can be used for this.

[hwchen]

Thanks for bringing this issue up, but I'm not very familiar with the flatpak sandbox. I'm happy to take a PR on this and learn more about it, I'm just pretty limited on bandwidth these days.