Human Readable Names Support

https://github.com/bitcoin/bips/pull/1551 describes a mechanism for mapping human-readable names (in the `Buser@domain` form) to BIP 21 URIs (targeting mostly silent payment addresses and BOLT12 offers). It would be nice if BDK came with support built-in for doing the lookups (or at least verifying DNSSEC proofs using https://git.bitcoin.ninja/index.cgi?p=dnssec-prover) and mapping them to a BIP 21 URI (and then resolving that to payment instructions).

We should probably have a discussion about where this kind of general payment instruction lookup logic should live - maybe LDK should refactor its BOLT12 offer and BOLT11 invoice parsing code into crates that don't depend on `lightning` so that we can have some generic "bitcoin payment instruction resolver" crate that BDK and LDK can use to resolve human readable names/BIP 21 URIs/bitcoin addresses to a list of payment instructions.