Support artifact uploads.

[asford]

Ensuring that the build directory is available within the plugin execution environment an unresolved problem. (buildkite-plugins/docker-compose-buildkite-plugin/pull/157, #7) and will likely require configuration at the agent level 😢.

It may be best to support reporting via build artifacts, utilizing the artifact store as to avoid mounting the build directory. This should mirror the annotate artifact embedding uri syntax. (eg. artifact://test_summary.md)

It would be real dope if we pre-processed the incoming markdown to convert artifact:// links into public links a la buildkite-agent annotate for embedded reporting.