Add optional support for YAML as a general request payload

[banool]

This PR adds support for YAML, in all the same ways that there is support for JSON. For the most part I just copied what we do for JSON, but with serde_yaml, which is mostly a drop in replacement.

In poem itself this is an optional feature, but given we already import serde_yaml in poem-openapi, I made it just include the YAML payload stuff by default.

Still a work in progress.

[banool]

I'm going to close this PR, I did some research and it seems like using YAML for payloads is generally frowned upon.

[sunli829]

I actually think it's useful, although there aren't many scenarios that use it and it seems to be close to completion?