Add a /api/v3/enrollments/ API with reduced footprint

[rhysyngsun]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/9559

Description (What does it do?)

This adds a /api/v3/enrollments/ API that is read-only for now and returns a very limited amount of information for now to keep it performant.

It also reworks the fixtures to make them easier to configure and to give guarantees of how many enrollments a user will get (previously it could be as few as 1 which made it harder to detect N+1 errors).

How can this be tested?

Tests should pass, you should be able to access the API /api/v3/enrollments and it return similar data to `/api/v2/enrollments, but with some problematic fields removed and much more performant.

[github-actions]

OpenAPI Changes

Show/hide ## Changes for v0.yaml:

## Changes for v0.yaml:
4 changes: 0 error, 0 warning, 4 info
info	[endpoint-added] at head/openapi/specs/v0.yaml	
	in API GET /api/v3/enrollments/
		endpoint added

info	[endpoint-added] at head/openapi/specs/v0.yaml	
	in API POST /api/v3/enrollments/
		endpoint added

info	[endpoint-added] at head/openapi/specs/v0.yaml	
	in API DELETE /api/v3/enrollments/{id}/
		endpoint added

info	[endpoint-added] at head/openapi/specs/v0.yaml	
	in API GET /api/v3/enrollments/{id}/
		endpoint added



## Changes for v1.yaml:
4 changes: 0 error, 0 warning, 4 info
info	[endpoint-added] at head/openapi/specs/v1.yaml	
	in API GET /api/v3/enrollments/
		endpoint added

info	[endpoint-added] at head/openapi/specs/v1.yaml	
	in API POST /api/v3/enrollments/
		endpoint added

info	[endpoint-added] at head/openapi/specs/v1.yaml	
	in API DELETE /api/v3/enrollments/{id}/
		endpoint added

info	[endpoint-added] at head/openapi/specs/v1.yaml	
	in API GET /api/v3/enrollments/{id}/
		endpoint added



## Changes for v2.yaml:
4 changes: 0 error, 0 warning, 4 info
info	[endpoint-added] at head/openapi/specs/v2.yaml	
	in API GET /api/v3/enrollments/
		endpoint added

info	[endpoint-added] at head/openapi/specs/v2.yaml	
	in API POST /api/v3/enrollments/
		endpoint added

info	[endpoint-added] at head/openapi/specs/v2.yaml	
	in API DELETE /api/v3/enrollments/{id}/
		endpoint added

info	[endpoint-added] at head/openapi/specs/v2.yaml	
	in API GET /api/v3/enrollments/{id}/
		endpoint added

Unexpected changes? Ensure your branch is up-to-date with main (consider rebasing).

[gumaerc]

@annagav I generated the API client library against this branch and loaded it into my instance of Learn. After switching everything over to use the V3 enrollments endpoint, I noticed no differences versus V2. I'm sure there are performance improvements, but but it wasn't super noticeable as I recently wiped my workstation and don't have a ton of test data. I'd say the best way to test this is to get it out into the wild, then put up another PR to switch Learn over to it and we can test in RC / production against larger data sets. Either way, all the data we need to get in Learn is included here so we don't need to add anything else at the moment.