[TASK] Pass in anchor bridge length to the APIs so that we can switch on different bridge sizes

[drewstone]

Overview

Currently we only support a bridge with 2 anchors. We need to generalize so that we can support more sizes. We already support passing in the proving key arbitrarily for different sizes.

https://github.com/webb-tools/webb.js/blob/master/packages/wasm-utils/src/proof/anchor.rs#L31