Add preload functionality and device specific config generation to generate balenaOS image for offline updates

[ab77]

preload

e.g. https://github.com/balena-io-playground/offline-updates#create-and-preload-release

config generation

e.g. https://github.com/balena-io-playground/offline-updates#configure-balenaos-image

This would probably be available on the device context, with ability to specify app/release id to preload as well as supply additional config.json values (ideally option to select from all supported).

[jellyfish-bot]

[cywang117] This issue has attached support thread https://jel.ly.fish/293222b7-2544-4b4e-92a0-6e61f5dd647e