fix: compress OTLP profile uploads

[arnabnandy7]

Summary

Fixes #346 by compressing OTLP profile uploads with gzip.

• Reuses the existing compressionLevelJFR configuration.
• Adds Content-Encoding: gzip when compression is enabled.
• Preserves application/x-protobuf as the content type.
• Keeps NO_COMPRESSION behavior unchanged.
• Updates the exporter test to verify the header and decompressed payload.

Testing

./gradlew :agent:test --tests io.pyroscope.javaagent.impl.PyroscopeExporterTest --rerun-tasks

Result: BUILD SUCCESSFUL

[arnabnandy7]

@korniltsev-grafanista @korniltsev to review

[arnabnandy7]

@korniltsev-grafanista @korniltsev please re-run the failed tests, and there are 3 other reviews need your attention. let me know if you can share your inputs there as well. Thanks again for your review and approval.