Difficulty length check

[AionJayT]

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Description

Please include a brief summary of the change that this pull request proposes. Include any relevant motivation and context. List any dependencies required for this change.

• The PR is mainly for adding difficulty length check in the header rule to prevent adversary padding big
  date into this field.

Fixes Issue # .

Type of change

Insert x into the following checkboxes to confirm (eg. [x]):

• Bug fix.
• New feature.
• Enhancement.
• Unit test.
• Breaking change (a fix or feature that causes existing functionality to not work as expected).
• Requires documentation update.

Testing

Please describe the tests you used to validate this pull request. Provide any relevant details for test configurations as well as any instructions to reproduce these results.

Verification

Insert x into the following checkboxes to confirm (eg. [x]):

• I have self-reviewed my own code and conformed to the style guidelines of this project.
• New and existing tests pass locally with my changes.
• I have added tests for my fix or feature.
• I have made appropriate changes to the corresponding documentation.
• My code generates no new warnings.
• Any dependent changes have been made.

[AlexandraRoatis]

with some minor comments

[aion-kelvin]

Looks good. Just have one JavaDoc-related request.