Esplora and electrum should allow producing an update based on outputs rather than script pubkey

[LLFourn]

Right now the only way to find out if an output is spent is to query the APIs for a script pubkey. What if I only have an outpoint that I am interested in or I just want to check if a transaction is confirmed.

In addition to the iterator of script pubkeys we currently pass in we should allow iterators for:

1. OutPoint which would return the tx with the outpoint on it (if it can find it) and any tx spending from it
2. Txid which would return the transaction with this txid in the update if it exists.