Add axum_extra::json!

[SabrinaJewson]

Motivation

Closes: #2937

Solution

Add axum_extra::json!, a macro that forwards to typed_json::json! to produce an ad-hoc JSON response with zero allocations.

I chose to put this in axum-extra rather than core Axum for two reasons:

1. It adds an extra dependency on typed-json. It’s a lightweight dependency and not exposed in the public API, but a dependency nonetheless.
2. It allows for json! to return an ErasedJson. Using Json<impl Serialize> isn’t ideal because if the json! macro borrows data, the type can’t be returned from a function. Using Response directly isn’t ideal because we lose Clone.

I had to modify a couple lines in the Cargo.toml of axum-extra in order to get the tests to compile on my machine – just enabling some feature flags.

[jplatte]

Thanks!