[POC/WIP] Add optional Bip352 silentpayments index
This PR proposes a new --silent-payments-index option that creates an index of Bip352 silent payments.
Most of the code is based on @Sosthene00's fork, but I made the following changes:
. It uses https://github.com/bitcoin-core/secp256k1/pull/1519 and https://github.com/rust-bitcoin/rust-secp256k1/pull/721 instead of the rust-silentpayments crate.
. It adds a new --silent-payments-index configuration option. Without this option, the server does not index silent payments and works the same as it does today.
. The code does not use Box[u8] in db.rs.
To run this, something like the following command can be used: cargo run -- --log-filters=INFO --db-dir <db_dir> --daemon-dir ~/.bitcoin/ --network signet --electrum-rpc-addr="127.0.0.1:60001" --silent-payments-index
does not index silent payments
but I can make silent payments to the node, from a client?