rustwitness: unify the interface with witnesscalc-adapter

[KimiWu123]

Currently the witness generation interface of rustwitness and witnesscalc-adapter are,

// rustwitness
type RustWitnessWtnsFn = fn(HashMap<String, Vec<BigInt>>) -> Vec<BigInt>;
// witnesscalc
type WitnesscalcWtnsFn = fn(&str) -> anyhow::Result<Vec<u8>>;

We should unitfy the interface of rustwitness to &str because it's more extendable and won't be limited by the input format (for example, zupass's input is a double array key:[[values],[values], ...] which can't fit in rustwitness).