Rewrite get_header relay communication to support SSZ and JSON

content negotiation. Includes:

- SSZ and JSON encoding/decoding for relay get_header responses
- Content-Type and Eth-Consensus-Version header parsing
- Accept header forwarding to relays with q-value ordering
- MIME parameter tolerance on relay response Content-Type
- Fork-aware SSZ bid value extraction for all supported forks
- Mock relay and validator SSZ support in test infrastructure
- get_header integration tests for both encodings
- Dynamic port allocation in tests via get_free_listener

Author: JasonVranek