add `source.pointer` support

[Roanmh]

from the atomic operations spec

The error object SHOULD include a source member with a pointer to the invalid operation

While not a requirement, this is effects the usability of the API greatly. Currently a validation error looks like this:

{
	"errors": [
		{
			"detail": "This field is required.",
			"status": "400",
			"code": "required"
		}
	]
}

The missing pointer makes this error very unhelpful.

Developer Notes

I think there is a simpler way to maintain the interface of get_serializer(): If AtomicOperationView stores the current index (and related arguments) in an instance variable, then get_serializer() can be called without arguments. get_serializer() can reference the instance variables instead of the arguments to determine the answer. We implemented this by overwriting get_serializer() in our subclass.

However, the pointer values will be incorrect. The Serializer for the given operation will generate a pointer without the atomic operation context in mind. e.g. /data/attributes/name instead of /atomic:operations/0/data/attributes/name. For our project we worked around this by implementing a custom exception handler. This callback can access the AtomicOperationsView instance, so we could get the correct index. Perhaps this library can write a thin wrapper around the json-api exception handler to make this correction? I don't know how custom exceptions handlers work with libraries like this.

The matter seems non-trivial for two reasons, 1) as shown below django-rest-framework-json-api does not support any arguments for ExampleView.get_serializer() and 2) the pointer must be adapted to the schema of the atomic operations extension e.g. /{ATOMIC_OPERATIONS}/{idx}/data/<path/to/error>

Field information is captured in the serializer.ValidationError at first, but format_drf_errors looses that information. format_drf_errors calls context["view"].get_serializer() without any arguments, which throws this error (captured via pdb):

(Pdb) context["view"].get_serializer()
*** TypeError: AtomicOperationView.get_serializer() missing 3 required positional arguments: 'idx', 'operation_code', and 'resource_type'

It then falls back to not providing a pointer.

My first guess is that this library needs its own exception_handler, but this would duplicate a lot of logic. I'm not sure how else to approach this without an upstream change to support per-error/per-field arguments on get_serializer(). Perhaps that is a possibility? I'm unfamiliar with the space.

Our use-case doesn't require this improvement yet, so I have to leave it here.