Make Datum/Redeemer method generics on LedgerClient instead of on the whole trait

[MitchTurner]

This will make it more flexible.

Essentially, the signature is currently:

pub trait LedgerClient<Datum, Redeemer>: Send + Sync {
    ...
    async fn outputs_at_address(&self, address: &Address)
        -> LedgerClientResult<Vec<Output<Datum>>>;
    ...
    async fn issue(&self, tx: Transaction<Datum, Redeemer>) -> LedgerClientResult<()>;

and we can make it

pub trait LedgerClient: Send + Sync {
    ...
    async fn outputs_at_address<Datum>(&self, address: &Address)
        -> LedgerClientResult<Vec<Output<Datum>>>;
    ...
    async fn issue<Datum, Redeemer>(&self, tx: Transaction<Datum, Redeemer>) -> LedgerClientResult<()>;