feat: expand API surface coverage

[darioAnongba]

Summary

Audit BDK wallet public API and identify unwrapped methods. The current wrapper covers core wallet operations but many useful APIs are missing.

Potential Areas

• Coin selection: Custom coin selection strategies
• PSBT utilities: combine, merge, extract_tx, more Psbt methods
• Descriptor templates: Standard descriptor templates (BIP44/49/84/86)
• TxBuilder options: add_utxo, add_foreign_utxo, set_exact_sequence, enable_rbf, etc.
• Wallet introspection: More query methods, spending policies

Tasks

• Audit bdk_wallet::Wallet public API and list all unwrapped methods
• Audit bdk_wallet::TxBuilder public API
• Prioritize by consumer demand and MetaMask Snap usage
• Implement wrappers in focused PRs (grouped by area)