Gzip agent api request bodies before sending them off

[moskyb]

This PR introduces support for gzipping agent API requests to improve the performance of pipeline uploads in low-bandwidth environments. Compression is opt-in so existing deployments will continue with uncompressed requests unless explicitly enabled.

• Added Compression: API client compresses requests when flag is enabled (disabled by default)
• Added CLI flag: --gzip-api-requests with environment variable BUILDKITE_GZIP_API_REQUESTS