fix(cubejs): direct Arrow passthrough with field mapping header

[acmeguy]

Summary

• keep native ClickHouse Arrow passthrough enabled for /load exports even when Cube aliases differ from semantic member names
• attach a base64url JSON response header that maps Arrow field names back to semantic member names
• expose the mapping headers to browser clients

Validation

• node --test test/loadExport.test.js test/arrowSerializer.test.js test/csvSerializer.test.js test/loadExportCapabilities.test.js

Notes

• this is the follow-up needed to make native Arrow passthrough viable without rewriting Arrow schema bytes

[acmeguy]

Replaced by #24, which is the same follow-up on a clean branch based on current main.