This pull request introduces multipart upload support for artifacts, allowing large files to be uploaded in multiple parts to S3, and adds end-to-end tests for these new endpoints. The changes span Nginx configuration, Lua signature computation logic, access restrictions, and comprehensive test coverage.

Multipart upload feature additions:

• Added four new Nginx locations in nginx.conf.template to handle multipart upload lifecycle: initiate, upload part, complete, and abort. Each endpoint proxies requests to S3 with proper validation and signature computation.

• Updated the Lua S3 signature computation logic to support multipart operations, including explicit resource canonicalization for each multipart endpoint and signature mode handling. [1] [2] [3]

Access control updates:

• Extended GitHub upload restriction paths in github_access.lua to include /upload-multipart/, ensuring proper access control for the new endpoints.

Testing improvements:

• Added a new end-to-end test suite in test_multipart_upload.py covering multipart upload scenarios: single-part, multi-part, abort, out-of-order part uploads, and validation for non-staging builds.