feat: Add from_body to Deserializer

[LucasPickering]

This exposes a method to create an HCL Deserializer directly from a Body. I know the from_body global fn already exists, but I specifically need a Deserializer so I can pass it to serde_path_to_error::deserialize.

I know Body already implements IntoDeserializer, but that creates a NewtypeStructDeserializer<Vec<Structure>> which doesn't seem to work for deserializing to arbitrary Deserialize types.

I didn't add any tests to this because it's so trivial. Happy to write one though if you'd prefer to have it.

[martinohmann]

That addition makes sense, thanks!