Refactor RPC methods to use HexData and an error constructor

[arya2]

Motivation

Multiple RPC methods are currently accepting hex-encoded data and similarly constructing ServerErrors.

Using the HexData type and a server error constructor will reduce repetition and improve maintainability.

Potentially something like:

{
    ...

    ready().await.map_server_error()?;

    get_height().ok_or_server_error("No blocks in state")?;
}

impl<T, E> MapServerError for Result<T, E> where E: Debug {
    fn map_server_error(result: Self) -> Result<T, jsonrpc_core::Error> {
        result.map_err(|error| jsonrpc_core::Error {
            code: ErrorCode::ServerError(0),
            message: format!("{error:?}"),
            data: None,
        })
    }
}

impl<T> OkOrServerError for Option<T> {
    fn ok_or_server_error<S: ToString>(option: Self, message: S) -> Result<T, jsonrpc_core::Error> {
        option.ok_or(jsonrpc_core::Error {
            code: ErrorCode::ServerError(0),
            message: message.to_string(),
            data: None,
        })
    }
}

Originally posted by Teor

[teor2345]

I've added these as TODOs to the code instead, so we do them when we are rewriting particular RPCs.