Define and Create a Model to store Content Signatures

[gerrod3]

author: rochacbruno

original redmine issue: https://pulp.plan.io/issues/9543

---

Story:

Ansible collections and Container Images can be signed using pulp signing services, once a signature is generated we need a place to store the signature along with its metadata.

Signatures are ASCII text blobs or encoded json data

There is no definition yet on how this Model must look like, something that has the following fields.

class ContentSignature(BaseModel | BaseContentType):
    signature_artifact: FilePath
    # or
    signature_blob: Text
    metadata: JsonField
    content_ptr: (reference to an artifact that can be Collection, Container Image etc)

A content artifact AnsibleCollectionVersion or ContainerImageManifest can have a relationship with one or many signatures