Boilerplate for community-contributed future backends

I have a strong personal interest in contributing to new `future` backends, particularly `clustermq` (https://github.com/HenrikBengtsson/future/issues/204) and AWS Batch (https://github.com/HenrikBengtsson/future/issues/423, https://github.com/wlandau/targets/issues/152). It would be great if there were an automated way to create boilerplate structure (similar to `usethis::create_package()` or `rstantools::rstan_create_package()`) and allow devs like me to easily plug in what it means for a future to initialize, resolve, and return a value. Combined with [`future.tests`](https://github.com/HenrikBengtsson/future.tests), I think this could be a powerful way to encourage new community-contributed backends.

Start by reviewing the future backend initialization, resolution, and value-return entry points, then compare them with the future.tests project and the linked clustermq and AWS Batch discussions. Done would mean a defined, automated boilerplate workflow that lets contributors create and implement new community backends.

Written by the indexing model from the issue text.