Refactor Task's created_resources field to use an ArrayField or HStore

From task query optimization PR: https://github.com/pulp/pulpcore/pull/3712:

created_resources currently uses a GenericReleationModel to store references to items created during a task. This can cause significant performance issues when quering tasks especially for relations that require Master-Detail related lookups. Also, when the associated resource gets deleted the task is left with a reference to null when serialized.

The logic around created_resources could be simplified if switched to a ArrayField or HStore, there would be no need for complicated lookup logic when quering tasks. Things to note for the conversion:

• The CreatedResource model is heavily used in tasking code and in plugins, usually like so: CreatedResource(resource).save(),
• created_resources is queriable through the REST API
• Current created_resources that point to null are mostly recoverable if they were not pointing to a RepositoryVersion

Start by tracing CreatedResource(resource).save() through the tasking code and the created_resources REST API, then review task query optimization PR #3712. Done means converting the storage while preserving existing task references, plugin usage, and API querying, including handling resources that now serialize as null.