Add collection download count to pulp ansible.

As a collection author, I would like to know how many times my collection has been downloaded.

Models

Create a CollectionVersionDownload model that tracks downloads by client. client and collection_version keys should be unique together:

Fields:

• collection_version: FK to collection version
• client: string. "pulp", "ansible-galaxy", or "other".
• count: int

This model should subclass BaseModel from pulpcore.

Note: Collection version downloads are tracked in a separate table instead of adding them as a field on the CollectionVersion model itself because CollectionVersion is immutable. Download counts don't need to be preserved during sync and import/export operations, so they can be stored separately from the content they're connected to.

Views

On artifact download endpoint:

1. check request user-agent
   1. if pulp: client = pulp
   2. if ansible galaxy: client = ansible-galaxy
   3. if anything else: client = other
2. get or create new download count object with collection version and client.
3. Increase count by 1

Serializers

CollectionVersion and Collection serializers:

Add download_count field.

"download_count": [
    {"client": "pulp", "downloads": 0}
    {"client": "ansible-galaxy", "downloads": 0}
    {"client": "other", "downloads": 0}
] 

On the collection serializer, the download counts should be the sum of all the collection versions. So if a collection has two versions with 3 and 5 downloads from the pulp client, the collection serializer would return 8 downloads.

Add last_downloaded field that returns the time that the object was last downloaded:

"last_downloaded": "2022-04-22T16:36:58.249615Z"