Allow sending with other serialization formats

[ecton]

Currently, bincode is forced to be the way that payloads are encoded. From looking in the code, it doesn't seem like this is a hard requirement. I'd like a mechanism to be able to change the encoding used for structures.

For many crates, bincode is a great option, but for BonsaiDb, I want the API to use a self-describing format, not bincode. Rather than force others to use Pot, I'd prefer a mechanism for me to be able to provide a serialization/deserialization implementation through a new trait rather than fabruic requiring the serialization to be done via Serde.

This would give users the most flexibility, and if they want to use serde, they can pick which encoding format to use themselves.