Store timing files as measurement artifacts

[yoshifuminakamura]

Summary

• add a Measurement Artifact ingest route for result-linked artifact files
• upload referenced timing observation JSON files from send_results.sh
• show profile archives and timing observation files together on Result Detail while keeping timing artifacts off the public surface

Tests

• .venv/bin/python -m pytest result_server/tests
• bash scripts/tests/test_send_results_profile_data.sh
• bash scripts/tests/test_bk_timing_observations.sh
• bash scripts/tests/test_result_common_json_contract.sh
• shellcheck -S error -f gcc scripts/result_server/send_results.sh scripts/tests/test_send_results_profile_data.sh