feat: add accessor methods for common PaymentKind fields

[jgmcalpine]

Description

Addresses issue #730.

Added payment_hash() as requested, but also identified that preimage and secret are common fields shared across most PaymentKind variants. Added accessor methods for all three to unify the API.

Changes

• Added payment_hash(), preimage(), and secret() to PaymentKind.
• Refactored PaymentDetailsUpdate::from to use these new methods, removing boilerplate matching logic.
• Added unit test payment_kind_accessor_methods to verify behavior across all variants (Bolt11, Bolt12, Spontaneous, Onchain).

PR Tasks

• Added tests
• Ran cargo fmt
• Ran cargo test

[ldk-reviews-bot]

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