[EPIC] Sigstore integration

[ipanova]

Is your feature request related to a problem? Please describe.
Pulp Container registry can mirror, upload and serve atomic signature type. We should add support for https://github.com/sigstore/cosign signature type

• As a user, I can serve cosign type signatures #1165
• As a user, I can mirror cosign type signatures #1166
• As a user, I can upload images signed with cosign altogether with their signature #1167
• As a user, I can cosign images via Pulp Container Registry #1168

Note user stories are subject to change after scoping and requirements gathering.

Additional context
Add any other context about the feature request here.