[tf.data] Add experimental ability to override the function threadpool.

The purpose of this feature is to enable experimentation with differentiating
the CPU resources available to different stages of a `tf.data` pipeline. As a
concrete example, we might use the new feature to move all input-related work
from the inter-op threadpool onto a separate threadpool, leaving the inter-op
threadpool free to execute higher priority work (such as dispatching ops that
send tensors to an accelerator). The current implementation only allows users
to create fixed-size `tensorflow::ThreadPool` resources, but we could imagine
opening up this API to allow custom threadpools as well.

PiperOrigin-RevId: 186614315