As a user, I can upload a Maven Artifact into Pulp

[pulpbot]

Author: @dkliban (dkliban@redhat.com)

Redmine Issue: 4640, https://pulp.plan.io/issues/4640

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The user should be able to:

1. Upload an Artifact either using the Artifact API or Uploads API.
2. Create MavenArtifact using the MavenArtifact API. The user should be able to provide 'group_id', 'artifact_id', 'version' and 'filename'.

This should work very similar to how the File plugin handles upload.

Most likely, the plugin already supports this workflow. As part of this story, a functoinal test should be added that makes sure that upload works.

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