Encapsulate validate_header_regarding_parent inside SealedHeader

[tcoratger]

Changes Made

This pull request encapsulates the validate_header_regarding_parent function within the SealedHeader struct. The encapsulation is done to improve code organization and enhance encapsulation of header-related validation logic.

• Errors from validate_header_regarding_parent are mapped inside a SealedHeaderError enum.
• An error field SealedHeaderError is introduced inside ConsensusError enum, transparently wrapping SealedHeaderError
• All old unused ConsensusError error fields that are wrapped inside SealedHeaderError are removed from ConsensusError enum to clean up and better organize errors there.

Should close #6193.

Motivation

Encapsulating validate_header_regarding_parent within SealedHeader promotes a cleaner code structure by keeping related functionality within the same module. This encapsulation enhances the readability and maintainability of the codebase.

Impact

This change does not introduce any functional changes but improves the organization of the code. Existing functionality remains unchanged, and this encapsulation serves as a step towards better code organization.

[mattsse]

I like this a lot, this is much better than having standalone functions.

some naming nits otherwise lgtm

pending @Rjected

[mattsse]

lgtm, pending @Rjected

[Rjected]

LGTM, I like that this puts the header validation errors in a new type