Spec, OpenAPI: Define hex case for binary and fixed values (#18352)

[zhang-arvin]

Fixes #18352

The table spec and the OpenAPI spec disagreed on how binary and fixed values are written as hex strings. format/spec.md Appendix D said "Stored as a hexadecimal string" with the lowercase example "000102ff" and no case rule, while rest-catalog-open-api.yaml (BinaryTypeValue / FixedTypeValue) said "serialized as an uppercase hexadecimal string". Implementations already differ, and both cases can appear in existing table metadata.

This change makes the rule explicit and consistent:

• Readers must accept either case (lowercase values can already be written by existing implementations, so a stricter rule would break existing tables).
• Writers should emit uppercase, matching the OpenAPI schema and the reference implementation.
• Update the Appendix D example to uppercase ("000102FF") and record the reader/writer rule in the description.
• Add the same rule to the BinaryTypeValue / FixedTypeValue descriptions in the OpenAPI spec, and regenerate rest-catalog-open-api.py.