Replace f64 ZEC values in JSON-RPC output with a fixed-point decimal type

[str4d]

There are a bunch of zcashd JSON-RPC methods that return decimal ZEC values. Currently to return those with jsonrpsee and its serde interface, we use an f64, but that is fragile.

In zcashd we instead used an explicit fixed-point decimal string representation that was then encoded as a number. I'm not quite sure how to replicate this via serde, but we need to figure it out.