Change api format to be consistent with other rpc endpoints

[ckoopmann]

For historical reasons in the current version we use an api format different from the standard rpc encoding:

1. snake_case field names (instead of camelCase)
2. decimal encoded numbers (instead of hex encoding)

If we avoid these differences we could drop all of the code in src/rpc/types.rs

[alextes]

Let's use camelCase and hex then. If the relay breaks with client specs I'd rather it do the work to make it readable to clients like Reth.

Question: does Reth support SSZ encoding for the data we need to pass?