Implement BlockHeader builder pattern

[austinabell]

• The BlockHeader is really hard to construct because of all of the fields and dependencies needed to build it. This allows constructing only the necessary parts and defaulting most
• Was building off these changes but figure these can come in before because there is a chance there could be a conflict with what someone else is working on.

@dutterbutter let me know if you want me to make any more of those fields mandatory if it affects it in future work (can always change later though)

[GregTheGreek]

Nit: Can you consistently add white space