[ecash-wallet] Add fluent API for building and broadcasting txs

Summary:
We want a powerful and user-friendly API that exposes commonly used steps in tx processing.

Here we initialize such an API. The MVP includes basic support for SLP and ALP txs.

• We can get the utxos required to build an arbitary tx with the exported selectUtxos function, which supports different selection strategies to support Postage protocol
• We can build a tx (and get info about it) without broadcasting it, by calling wallet.tx(<instructions>).build().fee() or wallet.tx(<instructions>).build().size(); or the actual TxBuilder with wallet.tx(<instructions>).build().tx
• We can build and broadcast a tx with one call with await wallet.tx(<instructions>).build().broadcast()
• We can specify the outIdx of token outputs and have extensive validation against token spec about what works / does not work
• We have included a regtest proof of concept for a postage tx
• We have included regtest confirmed behavior for basic XEC, SLP, and ALP txs

This diff ended up piling in many features, as we had to build out a lot of this capability to properly evaluate the API. While the methods in this app do involve some complexity, it is greatly simplified compared to using ecash-lib directly, and the strong typing and validation enhance the developer experience.

From here,

• We should add a websocket so the user does not need to remember to call await wallet.sync() before sending every tx
• Support other token types
• Support Agora actions
• Add support for chained txs, i.e. await wallet.tx(<instructions>).build().broadcast() should "just work" even if the instructions are asking to send more token outputs than a single tx can hold, or more XEC outputs than the tx broadcast limit of 100kb will tolerate.

Test Plan:
npm test

integration tests:
build node in build dir, then, from bitcoin-abc/modules/ecash-wallet/, BUILD_DIR="${PWD}/../../build" npm run integration-tests

Review API, suggest different names, shapes of objects, anything that should go in now before we start to add more features

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Subscribers: alitayin, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D17822