Remove SSZ Union from BlockHeaderWithProof container type

[acolytec3]

Following this, it is likely we will need to remove BlockHeaderProofType union type and replace it with a simple ByteList.

This is roughly our todo list

• - remove BlockHeaderProofType and replace with a Bytelist with a MAX_PROOF_SIZE as defined in the spec
• - Switch logic in all of the header proof serializing/deserializing code in src/networks/history.ts to select the proof type based on the timestamp embedded in the block header
• - Create an enum containing the hardfork timestamps for Bellatrix and Capella so we use the right proof types
• - Update all the tests and test with the new test vectors from the above linked PR