Allow indexing outputs on transactions that contain a metadata tag

[KtorZ]

Discussed in #103

^{Originally posted by Quantumplation January 9, 2023}
With Sundae governance, we recently implemented on-chain voting; now, users can cast their vote "on-chain" (totally optional, we still support fee-less off-chain voting). This is helpful for people concerned about censorship, or more commonly, those who have hardware or CLI wallets where it's difficult to signData or interface with the web-app.

When it comes time to tally the votes for a given proposal, we then add up the users balances for all of the off-chain votes (already using kupo); and now we need to identify the relevant on-chain transactions to add into the mix.

For this, it would be super useful to be able to specify something like

--matches "meta:31415"

Then, any transactions (or, more accurately, the outputs of that transaction) that have the metadata tag 31415 get indexed and then are queryable.

If this doesn't fit with the design of kupo, this might make sense as an ogmios feature (chainsync, but only tell me about txs that match some pattern), or even it's own standalone tool in the same spirit as kupo.

Obviously, this wouldn't be too hard to implement directly on ogmios ourselves, so if this turns into a "will not do" that's fine, just thought I'd throw it out there :)