Add experimental silent payment transaction creation support

[nymius]

Description

This PR adds experimental support for creating silent payment transactions through a new CreateSpTx command. The implementation integrates the bdk_sp crate to enable sending Bitcoin to silent payment addresses.

Key changes:

• Adds bdk_sp dependency as an optional feature
• Implements CreateSpTx command with support for silent payment recipients
• Includes parser for silent payment address:amount pairs
• Returns signed transactions ready for broadcast (not PSBTs due to silent payment derivation constraints)

Notes to the reviewers

• This feature is marked as EXPERIMENTAL and includes warnings against mainnet use
• The command returns signed transactions directly rather than PSBTs because silent payment script pubkey derivation cannot be performed securely in a trustless manner with standard PSBT workflows
• RBF is disabled for silent payment transactions (sequence set to MAX)
• The implementation handles both single key and extended key signers
• Error handling uses temporary .expect() calls that should be addressed in future iterations

Changelog notice

Added: Experimental silent payment transaction creation via CreateSpTx command (feature-gated behind sp flag)

Checklists

All Submissions

• [x] I've signed all my commits
• [x] I followed the conventional commit guidelines
• [x] I ran cargo fmt, clippy and test before committing

New Features

~~* [ ] I've added tests for the new feature~~

• [x] I've added docs for the new feature