Add support for background pushes that set content-available

[buddhisthead]

This would be similar to the option to generate the mutable content flag.
Within the aps section, add "content-available" : 1. This should not be included with other keys in the 'aps' section, but can include other data which may also need support for setting.

[AvdLee]

@buddhisthead feel free to open a PR! I'm pretty sure it will be an easy one and it would be awesome to have you contributing to the project 💪🏻

[github-actions]

This issue is stale because it has been open for 30 days with no activity. Remove the Stale label or comment or this will be closed in 10 days.

[github-actions]

This issue is stale because it has been open for 30 days with no activity. Remove the Stale label or comment or this will be closed in 10 days.