Diagnostic data: sanitize swap and reverse swap infos

[ok300]

This PR ensures our diagnostic data doesn't include sensitive fields for swaps and reverse swaps.

[ok300]

@JssDWt @roeierez I simplified the PR based on the feedback above: c9dc874

It now:

• allows for both sanitized serialization (used in generate_sdk_diagnostic_data) and preserves the default non-sanitized serialization (used elsewhere, like when reconstructing SwapInfos as part of list_payments)
• defining the sanitization logic doesn't need to be updated when new fields are added to the underlying struct (only the sanitized fields have to be explicitly stated)
• defining the sanitization logic is simple to do for new structs (just implement Sanitize<T>)

What do you think?

[JssDWt]

Just a nit, otherwise LGTM