feature: Add block header field metadata

[marioevz]

Changes Included

• Replaces the fragile dictionary mixing method to collect the header fields with a method based on field metadata, and now each header field contains metadata with the following information:
  • Method used to determine requirement based on block number, timestamp and fork configuration
  • Field source from: Current test environment, or transition tool (t8n) result dictionary
  • Default value if field is required but no value was provided by either the test environment or the transition tool result

Open questions:
Should we throw an exception if the t8n tool did not provide us the sufficient information to create the block?
At the moment we provide defaults for the 4844 fields, but for 4788 the beacon block root contained in the header will actually modify the state root, and therefore we should throw an exception in this case if the field was not provided or was ignored when calculating the state root.

Fixes #215

@winsvega

[winsvega]

Ideally yes it should response all the fields indicating t8n knows what it's doing

[spencer-tb]

LGTM! 🚀