Merge bitcoindevkit/bdk#689: Add allow_dust method to TxBuilder

bfd7b2f65debe57edc5d563c071f0bfce592a8d2 Allow creating transactions with dust outputs (Liam)

Pull request description:

  We needed this for testing our wallet with dust, does this look like a reasonable feature? If so, I'll go ahead and add a test, update the changelog, etc.

ACKs for top commit:
  danielabrozzoni:
    tACK bfd7b2f65debe57edc5d563c071f0bfce592a8d2

Tree-SHA512: b467b365d8a68f5a868cc5cc88387677533e8fb0bf543bf4c7a5b984f8b28972281029a3be8d2c92cee7d6ee05c243d12af0841e7a7e1d652745567557f2bede