feat: ergonomic improvements to HTTP payload deserialization

[mlaota]

📬 Issue #, if available:
#917

✍️ Description of changes:
Ergonomic improvements to HTTP payload deserialization with intent-driven helper methods (json and form_url_encoded are the initial implementations). Details in RFC (#917).

Note: there was a bug in RequestPayloadExt::payload where if the content-type header was present but the body was empty, it would try to parse it anyways, resulting in a PayloadError. The bug was fixed when I updated the function to delegate to json and form_urlencoded. I added a couple unit tests in this PR for that scenario to ensure the behavior doesn't regress if its implementation drifts from the new functions in the future.

🔏 By submitting this pull request

• I confirm that I've ran cargo +nightly fmt.
• I confirm that I've ran cargo clippy --fix.
• I confirm that I've made a best effort attempt to update all relevant documentation.
• I confirm that my contribution is made under the terms of the Apache 2.0 license.

[calavera]

Thanks for adding all those tests and new documentation! ✨

[calavera]

I believe this is not used, can you remove it? We also have a similar type already exposed in lambda-http/lib.rs.

[mlaota]

Good catch! Removed.

(I was considering a generic Other variant for the new error types that would keep a box error. Decided to keep the errors as a single-variant enum to leave the option open without forcing users to handle an error path would have been currently impossible, but let me know if that's something you'd actually prefer)

[calavera]

Please, remove unused types.