Format timestamps that have a timezone using RFC339 format #599

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I have timestamps from UTC and I want to format them in RFC3339 format, such as 2021-07-20T23:28:50Z

Right now, the only formatting supported by Arrow is "naive" timestamps (aka without any timezone) format, such as 2021-07-20 23:28:50 (note missing Z)

Describe the solution you'd like
I would like to to be able to specify that a timestamp is printed with the specified embedded timezone, ideally in RFC3339 format

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
It is not even possible to create arrays that hold such timestamps yet. See #597 for details