Add Kaitai Struct definitions for block header digest items

[FlorianFranzen]

This will still need additional work before we can merge this:

• Sync math and kaitai definitions, especially around the 4-byte ASCII identifier
• Use consistent naming, e.g. consensus message vs. post runtime digest.
• Use digest parsers in block parsing test
• Add binary example blocks that cover all header digest types
• Resolve remaining TODOs