Three-hop blinded paths

[jkczyz]

Work in progress split out from #2781.

Summary by CodeRabbit

• New Features

  • Enhanced payment relay capabilities with new conversion logic.
  • Implemented additional functions for feature management and route blinding.
  • Added new tests for BOLT 12 Offers payment flow.
  • Introduced improvements to the onion messaging system, including new functions for message handling.

• Enhancements

  • Improved timestamp handling with conditional compilation.
  • Expanded node configuration with new messaging and entropy structures.

• Refactor

  • Reorganized import statements and reordered struct declarations for better code clarity.
  • Updated conditional compilation attributes to refine feature-dependent code execution.
  • Refined the payment and refund process with new methods and adjustments to existing ones.

• Bug Fixes

  • Adjusted access levels and conditional compilation directives to fix potential issues with routing and gossip logic.

• Documentation

  • No visible changes to end-users.