Compatibility with Poison 5.0 and/or other JSON decoders/encoders?

[michallepicki]

Currently the google_gax project depends on poison >= 3.0.0 and < 5.0.0. This means that we cannot use Poison 5.0 without overriding the version. https://github.com/googleapis/elixir-google-api/blob/92bcdea0ea6d43b985d48fcd18d3b0bd23017d8b/clients/gax/lib/google_api/gax/connection.ex#L38

Would it be possible to check if poison 5.0 causes problems, and if not, loosen the version requirement to allow Poison 5.0?

Alternatively, would it be possible to remove this dependency (or mark as optional) and allow the user to configure a different JSON decoder/encoder, like e.g. Jason? Tesla is configurable so it should be rather easy. In fact I think some of the clients already use Jason because it's the default in Tesla, e.g. here: https://github.com/googleapis/elixir-google-api/blob/92bcdea0ea6d43b985d48fcd18d3b0bd23017d8b/clients/task_queue/lib/google_api/task_queue/v1beta2/connection.ex#L29

[esambo]

FYI, see the related #8251