[XLA:AOT] Add support for resource variables as inputs/outputs

This is what I think is the minimum viable solution and also serves as an RFC
for the direction in which this API should evolve.

With this change resource variables behave exactly the same way as regular
input/outputs, any variable gets translated into a pair of argument/result
accessors. Initial values coming from the graph are ignored, the user has to
supply their own instead.

For the configuration this requires specifying name/shape/type for all
variables used in the model. I tried to infer the shape from the graph but it's
rather annoying because the arguments are create very early in the process.

This is intentionally lacking documentation, setting the API in stone now seems
too early. It's unclear to me whether we want to keep the argument/result
matching long-term or switch to something that supports in-place updates. The
current API cannot do that because it would violate const correctness.

PiperOrigin-RevId: 233042296