Upgrade to bdk v0.28

[BitcoinZavior]

Updated bdk flutter package to align with upgrades to BDK FFI version 0.28

Fixes #60

APIs added

• Expose Address class's payload and network functions
• Add fromScript constructor to Address class
• Add SignOptions to Wallet.sign() params.
• Add jsonSerialize function to PartiallySignedTransaction, to get the JSON serialized value of all PSBT fields.
• Expose Transaction class's txid, weight, size, vsize, isCoinBase, isExplicitlyRbf, version, isLockTimeEnabled, lockTime, input and output functions.

[BullishNode]

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