LSPS2: Promise generation and validation

[johncantrell97]

The spec mentions the ability for an LSP to add additional information into the promise. It also talks about the intentional vagueness in how this should work. Perhaps we want to design this in a way similar to LDK's KeysManager where we provide a reasonable default implementation but allow for flexibility should the LSP need it.

This feels really low priority at the moment but just creating issues for things in the spec we are currently missing.

[tnull]

Agree, the vagueness is one reason why we should keep the LSP-specific promise generation and validation logic seperate from the message types/serialization formats, i.e., https://github.com/lightningdevkit/ldk-lsp-client/issues/30