Merge bitcoindevkit/bdk#1681: types: Make Utxo::Foreign::sequence not optional

Merge bitcoindevkit/bdk#1681: types: Make Utxo::Foreign::sequence not optional

d96dac426187379890fb84315a51d81f37bbd0dc types: Make Utxo::Foreign::sequence not optional (Steven Roose)

Pull request description:

  It's never kept internally as optional at this point.

ACKs for top commit:
  notmandatory:
    ReACK d96dac426187379890fb84315a51d81f37bbd0dc

Tree-SHA512: 03d72dbdb0b2b05e6e15d235e35d77bb5ed85dfa9f3bedc879a50edcd3cd22277624854c76b270bc4987f5c463bba735d6a10ac97da6aae75d503e5e1f816c6a