Multipart artifact upload

[DrJosh9000]

Description

Add a multipart-upload mechanism for Buildkite-hosted artifacts, and upload parts in parallel (with each other, and with other artifacts).

Context

https://linear.app/buildkite/issue/PIPE-180/implement-agent-half-of-multipart-uploads

Changes

The changes to artifact uploading is substantial, but basically:

• Some tidyups to the existing code, such as context propagation, un-exporting the Collect method
• FormUploader is renamed to BKUploader, since it will manage both form uploads and multipart uploads to BK
• Changing all the existing uploaders to return one or more units of work, which are ultimately responsible for performing an upload
• Running work units concurrently on a pool of worker goroutines, and communicating between goroutines using channels
• Actually implementing multipart uploads:
  • Alter the batch artifact creation API to signal that multipart is supported and enabled
  • Alter the creation response to handle per-artifact instructions and multiple actions per instruction
  • Alter the batch update API to accept part number-ETag mappings
  • Implement multipart upload work units that are responsible for uploading a single part each

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)

[CerealBoy]

LGTM!