[good first issue] Add `PaymentKind::payment_hash` as an accessor method

[TheBlueMatt-macbook]

nice to not have to match on several variants to extra the hash, probably a few other common fields could use it too.

[jgmcalpine]

Hi! I would like to work on this issue, I am setting up my environment and will submit a PR shortly.

[jgmcalpine]

I've implemented the accessor methods and opened a PR for review here: #734

[tnull]

I'm a bit on the fence regarding adding Lightning-specific accessor APIs for PaymentKind which is explicitly an umbrella type also covering other payment methods that don't feature payment hashes/secrets/pre-images.

[TheBlueMatt]

Just annoying to have to match every time you want to extract stuff, but doesn't really matter that much, if you think its gross, close.

[tnull]

Just annoying to have to match every time you want to extract stuff, but doesn't really matter that much, if you think its gross, close.

Yeah, still not convinced we should do this, and in #734 it turned out that we can't even easily expose it in bindings due to PaymentKind being an enum interface, disallowing to expose further methods. So, for the time being, I'll close this as wontfix. Maybe if uniffi changes some of the requirements we can revisit this if we still think the sugar would be worth it.