supporting silent payments (bip352)

Description

@MorenoProg asked about how to implement silent payments in https://github.com/spesmilo/electrum/issues/7408#issuecomment-2796696251

Some preliminary remarks:

• the "send to" logic does not require taproot, but the "spend from" logic does.
• actually, even if we do not expose it to users, we already have limited support for taproot key-spend (see #9082)
• electrum-ecc does not support musig yet, but recent libsecp256k1 does, so I guess musig could be added to electrum-ecc "easily".

Note that for a feature to be merged in Electrum, it must not create a situation where user funds are not recoverable from seed. AFAICS, receiving silent payments would not only need a taproot wallet, but they would also break that assumption. Thus, if you want to implement the full "spend from" logic, I think it should be a plugin.

Also, the receiver will need to scan the blockchain in a way that is currently not indexed by Electrum servers. So you need an extra channel to request that information. That's another reason to implement this as a plugin (maybe talking to a local bitcoind)

Start by reviewing the silent-payments requirements in BIP352 and the limited taproot key-spend support referenced in #9082. Then inspect electrum-ecc, its libsecp256k1 capabilities, and the plugin and local bitcoind integration points; done means an agreed design that preserves seed recovery and supports the required blockchain scanning.