Add convenience methods to EndpointSet

[grahamu]

What does this PR do?

Adds commonly used code to EndpointSet.prepare() to retrieve a Resource object if the Resource object model is based on Django ModelBase, and if the request method is appropriate.

Where should the reviewer start?

Read the updated code.

How should this be manually tested?

Run tests.

Any background context you want to provide?

In another project requiring pinax-api I noticed 75% of EndpointSets use almost identical logic to obtain a Resource object via their individual prepare() and get_queryset() methods. This update aims to relieve that tedium.

get_pk() and get_resource_object_model() allow for customizing the object model and PK before object retrieval. This helps when an endpoint proxies for other models, or when a PK needs translation before object lookup.

What ticket or issue # does this fix?

N/A

Definition of Done (check if considered and/or addressed):

• Is there appropriate test coverage?
• Does this PR require a regression test? All fixes require a regression test.
• Did you update internal documentation appropriately?
• Did you update (or alert the appropriate person to update) user-facing documentation?