As a user I can view container image signatures

[pulpbot]

Author: @ipanova (ipanova@redhat.com)

Redmine Issue: 9513, https://pulp.plan.io/issues/9513

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Provide an extension of the docker/distribution API.

Assuming a signed image has already been pushed into the Pulp Container Registry, one can read the signature(s)( any user with access to the image) using the following calls:

• GET /extensions/v2/<namespace>/<name>/signatures/
• GET /extensions/v2/<namespace>/<name>/signatures/<imagesha256digest>

See https://docs.openshift.com/container-platform/3.10/admin_guide/image_signatures.html#reading-image-signatures-via-registry-api and https://github.com/openshift/origin/pull/12504/files