Add msgpack wire protocol, vendor CMP library, and env-configurable data format

- Add MessagePack-based wire protocol (call_function_msgpack) in accel.c
  alongside the existing rowdat_1 protocol, with full type support
  including temporal types, vectors, geography, BSON, and decimals
- Vendor the CMP (C MessagePack) library by Charles Gunyon into
  vendor/cmp/ with LICENSE and README.md
- Update setup.py to compile vendor/cmp/cmp.c and add include path
- Make DATA_FORMAT configurable via EXTERNAL_UDF_DATA_FORMAT env var,
  defaulting to 'msgpack'
- Add format mismatch detection in call_function to give clear errors
  when client and server disagree on wire format
- Rename call_function_accel to call_function_rowdat_1 for clarity and
  export both protocol entry points from the C extension

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kesmit13