Add raw value support

[ethanfrey]

This is an interesting type from serde-json: https://docs.serde.rs/serde_json/value/struct.RawValue.html

It would be cool to support it, but no idea how much work it would be. This would let us actually embed json objects in other objects and not base64 encoded them (Binary) when embedding them, making a much nicer API for messages containing messages, like cw20 send.

To do it efficiently, it would also require us to remove the 'static constraints on from_slice and use an arbitrary lifetime 'a. (Until now we only support owned data)

The implementation seems a bit magic to me and does seem to require visit_map to be implemented... https://docs.serde.rs/src/serde_json/raw.rs.html#291-319 which probably would fail out of the box.