Skip json-encoding the HistoryEndpoint response

[carver]

I think these responses should just be in the native representation. Encoding them first seems like a waste, and creates ugly workarounds like in #908, where we have to decode the header back from json in order to mash it up with the block bodies and then re-encoding it.

It seems like it should be straightforward to do the json-encoding in the rpc/ crate, as needed.

It may be that after removing this encoding, we might want to fully strip out the json-encoding logic from our native types, and purely rely on the encoding logic of reth types. (Our implementation was buggy, anyway. I can't think of a good reason for us to maintain our own copy when someone else is already maintaining it.)

[carver]

I haven't actually started coding this, so maybe my opinion will change when I try. If @ogenev has any thoughts before I dig into it, I'm all ears!