support SSZ encoding

[ralexstokes]

right now this repo only supports the JSON codec for the API

although the beacon APIs support a SSZ codec for some methods

[chirag-bgh]

on it.

[ralexstokes]

@chirag-bgh happy to chat high-level strategy before you get too far

ideally we can keep most of the current client architecture as is and just support SSZ {de,en}coding for the methods that support it, where we can dispatch on Content-Type before proceeding down either way

[ralexstokes]

we should add some kind of Codec input when constructing a client, and then the client should use the input to support the more efficient encodings where possible