add scaffolding for executable python spec

[jacobkaufmann]

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the ethereum consensus specs are executable, such that the specs are testable, and test vectors can be generated for consensus clients to consume. the builder specs should be executable in a similar way.

the scope of this issue is to set up the basic scaffolding for executable builder specs.

for example:

• script to parse the markdown files and generate python files with defined constants, containers, functions, etc
• minimal test(s)
• mechanism to execute the test(s) (e.g. Makefile)

[jacobkaufmann]

cannot assign myself, but I'm working on this