Add pipelines artifacts API endpoint

It would be useful to be able to access the artifacts of a pipeline from the rest API. We want to write some automation that downloads artifacts from completed pipelines to test them on some physical devices within our network, but as far as I can tell from the API the artifacts are not exposed as part of the pipeline response.

Workaround

Artifacts can be uploaded to the Downloads section of the repository using the following pipe:

https://bitbucket.org/product/features/pipelines/integrations?search=upload&p=atlassian/bitbucket-upload-file

They can then be retrieved using a curl call with our API as follows:

curl -sL -u bb_username:app_password "https://api.bitbucket.org/2.0/repositories/workspace_id/repo_name/downloads/build.zip" --output "build.zip"