Move operationId and tags from openapi_overrides to Spectacular

Before

class MyEndpoint(RetrieveAPIView):
    openapi_overrides = {
        "operationId": "CustomId",
        "tags": ["elements"]
    }

After

@extend_schema_view(
    get=extend_schema(operation_id='CustomId', tags=['elements'])
)
class MyEndpoint(RetrieveAPIView):

For endpoints that have a def get(…): method, add the @extend_schema on it directly.

For endpoints that have multiple methods, adding @extend_schema(tags=['elements']) to the class adds tags to every method at once.

A simple way to detect where changes are still required is to change the list of supported keys to get warnings in manage.py spectacular.