manta.js: expose methods to get fee estimates for all private transaction types

[Kevingislason]

Currently, it's not possible to conveniently get accurate fee estimates using manta.js. Fee estimates require an extrinsic, and building a private transfer extrinsic requires a proof, which for end users requires approving a transaction through manta-signer. We can sidestep the need to interact with manta-signer by exposing a method to build "dummy" private transaction extrinsics, which have invalid but correctly shaped proofs / other parameters.

We could also create functionality to get public transfer fee estimates for convenience.