Add Silent Payment Support

[42Pupusas]

I'm working on the ELIP for Silent Payments and have a reference implementation working with LWK.

I have also started prototyping what an integration would look like for LWK to support SP directly:

1. Feature-gate: silentpayments feature, default-off, gates the module (uses bech32)
2. New lwk_common types: SilentPaymentAddress + SilentPaymentScanKeys
3. Key origin: add SwSigner::silent_payment_keys() deriving b_scan/b_spend from seed at an agreed BIP-352 path
4. Wire send: TxBuilder (coin selection + balancing around build_confidential_sp_txout)
5. Wire receive: scan-client trait SilentPaymentTweakClient into the blockchain clients
6. Test vectors: KAT vectors shared with the ELIP

A few open questions that came up during prototyping:

• Scan-client shape: extend Esplora/Electrum with a tweaks index vs standalone server?
• Send API surface: new TxBuilder method vs recipient-type variant?
• HW signing: software-signer-only for v0 acceptable? (Jade/Ledger don't have an API for tweaks)
• Labels scope: full label mechanism in v0, or change-label only?

Most of the changes are expressed in the reference implementation, but I would greatly appreciate guidance on of comments on the current plan an questions.