Description

This function allows the implementation of the sending flow in silent payments without moving that much from the usual PSBT creation flow.
To unlock the sending flow for silentpayments this should be the function to release first for external use.
Currently, it lacks unit test, as well as its upstream dependencies in the crate.

Goals

Implement unit test for the following functions:

• derive_sp
• get_smallest_lexicographic_outpoint
• extract_pubkey
• create_silentpayment_partial_secret
• create_silentpayment_scriptpubkeys