NUT-17: Simplify NUT-17 subscription kinds

[callebtc]

Implementations

• Cashu-TS: feat(nut17)!: method-neutral quote subscriptions cashu-ts#668
• CDK: fix: support generic NUT-17 quote kinds cdk#1988
• Nutshell: fix(websocket): support generic quote kinds nutshell#1013

Summary

• replace method-specific SubscriptionKind values with generic mint_quote and melt_quote
• keep proof_state unchanged
• update NUT-17 command examples to advertise quote subscriptions per method-unit pair

Rationale

The subscription kind is now independent of the payment method, which makes the websocket API extensible to custom payment methods without needing method-specific quote kinds in the enum.

[Egge21M]

I like it. However we should make sure that mints are fine with signaling support only on that high level instead of on a method level

[robwoodgate]

ACK. LGTM

[robwoodgate]

If we decide not to go this route for any reason, then we need to add online_mint_quote / online_melt_quote to the list, and remember to update it when new first class methods are added.

But with the companion PR below, this should allow mints and wallets to track existing and custom method quotes via websockets in a sane way:

• add method field to quote responses, closes #378 #387