WIP: Silent Payments (Send)

[reez]

Description

This is a draft for adding experimental silent payment send support to bdk-ffi in small steps.

• Add bdk_sp as an optional dependency behind experimental-silent-payments.
• Expose the smallest useful send API.

The feature stays off by default, so existing builds are unchanged while we work through the API.

Notes to the reviewers

• Before starting this the one bdk-sp change I wanted in place was bdk-sp #63. That fix is now included in bdk_sp 0.2.0.
• For now I’m mainly looking for feedback on the dependency and feature setup.

Documentation

• bdk_wallet
• bitcoin
• uniffi
• Other: bdk_sp 0.2.0

Changelog

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR