Issues with pydantic FixtureHeader validation

Using existing pydantic models issues:

- can't have default fields.
because we must verify that all required fields are provided in test json.

- conflict between hash and block_hash dynamic field
we compute hash value on request, but we must use the one provided by test in hash field. this must be one fields but perhaps can use a trick to set it to None and compute it on post validation step if it is None

- multiple aliases does not guarantee that generated test structure always follow same name format
seems like multiple aliases will allow the structure to have inconsistent naming format, when we expect tests to have fixed field names in structures