Consider providing more structure to backend_config in CustomCallOp

[burmako]

As we're working towards locking down compatibility, one important thing that comes to mind is providing an escape hatch for features that aren't yet ready for strong compatibility guarantees.

backend_config in CustomCallOp is a powerful escape hatch, but currently it's unstructured - it's just a sequence of bytes. #637 provides an inspiration for how to make this more convenience - turn backend_config into a dictionary instead.

[GleasonK]

Closing in favor of #637, which also suggests to add a dictionary for structure to CustomCall.