Add optional support for humantime Duration in poem Object

[banool]

I would love to be able to represent durations as part of the OpenAPI spec. As it is now you have to just make a field like "duration_secs" and trust the user will pass the value in as seconds. With this, you can instead get the rich parsing from humantime.

Naturally I made this an optional feature.

Tested with:

cargo test --features humantime

I see the other external types largely don't have tests so I didn't write specific tests for this either.

[sunli829]

Thank you, please use nightly Rust to format the source:

cargo +nightly fmt --all

[sunli829]

Thanks 😊