refactor(transaction): use BufferWriter

[OttoAllmendinger]

This mirrors the suggested upstream changes here: https://github.com/bitcoinjs/bitcoinjs-lib/pull/1533

Issue BG-17233

---

fc9f1ce

refactor(transaction): Use backported BufferWriter

Update BufferWriter to modified backported BufferWriter that is submitted here: https://github.com/bitcoinjs/bitcoinjs-lib/pull/1533

Backported files are from this branch: https://github.com/OttoAllmendinger/bitcoinjs-lib/tree/add-buffer-writer.backport-bitgo-utxo

These commands can be used to retrieve the filese:

add-buffer-writer.backport-bitgo-utxo git checkout FETCH_HEAD
src/buffer_writer.js test/buffer_writer.js

• Use more flexible interface by allowing buffer to be passed as argument
• Remove getBuffer() (use bufferWriter.buffer instead)
• Add tests

Issue: BG-17233