Define API to serve ContentSignatures

[gerrod3]

author: rochacbruno

original redmine issue: https://pulp.plan.io/issues/9544

---

Related to https://pulp.plan.io/issues/9543

Story:

As a client consuming a content, I want to be able to fetch all the signatures related to a specific content in order to perform verification prior to installation.

e.g:

GET /pulp/signatures/

{
"signatures": [
    ... all the signatures present for that specif content version or artifact
    ]
}

The exact design of that API serializer is TBD