Ability to print JSON payload in the event of an error

[AntoniosBarotsis]

This should be mostly useful for internal use (or for when submitting a bug report for the crate). The users of the crate shouldn't have to interact directly with the raw JSON ever since it is fully parsed into structs.

This won't add a new dependency since serde_json is already used by reqwest.

This is a low priority issue so I'll do this eventually™.

Checklist

• replace all the response.json::<SomeStruct>().await?; calls to a response.text().await? and serde_json::from_str::<SomeStruct>(...) calls
• Make sure to redact any sensitive information before saving to the error struct
• Maybe feature gate (or debug build gate?) this entire thing to not move giant strings around all the time