SDK: Improve result of build functions

[jurevans]

Currently, all of the build_ functions (including build_batch) in the @heliax/namada-sdk package return a Rust struct instance (wasm pointer). It would be better to return a deserialized value from the SDK calls so they can be more easily inspected and destructured. We could Borsh-serialize/deserialize the BuiltTx type, and deserialize when returning from the SDK

Example of what this value would look like:

import { WrapperTxProps } from "@namada/types";

// We'll use borsh schemas instead of type defs, but essentially what the user
// will see is this:

type AccountPublicKeysMap = {
  pkToIdx: [string, number];
  idxToPk: [number, string];
}

type SigningTxData = {
  publicKeys: string[];
  threshold: number;
  feePayer: string;
  accountPublicKeysMap?: AccountPublicKeysMap[];
  owner?: string;
}

type Tx = {
  // tx hash
  hash: string;
  // tx bytes
  bytes: Uint8Array;
  // args used to create args::Tx in namada-sdk
  args: WrapperTxProps;
  // serialized signing data bytes
  signingDataBytes: SigningTxData[];
}