RFC: Implementing multiple serialization methods

Sometimes you want to serialize a data class into multiple different data formats. The most common is JSON, but others include MessagePack, protobuf, XML, custom binary payloads, and more.

Serde (for Rust) has a list of data formats that are supported through external plugins. https://serde.rs/#data-formats

It would be nice to have some examples of how to handle serialization with built_value to multiple data formats.

I noticed we have `StandardJsonPlugin`, but it's not clear to me how we could use that and something like a MessagePackPlugin simultaneously, especially since `@BuiltValue` only supports String wire names, and it's not clear how we would input more complex information (especially in the case of binary formats). I suspect we'd have to duplicate the serializer stack and have different plugins on each one?

(I would have opened this as a discussion but I noticed it's not enabled on this repo!)