Add experimental `FunctionLibraryRuntime::InstantiateOptions::overlay_lib`.

This option makes it possible to instantiate functions from a library
that has been loaded separately from the runtime's own library. We
plan to use this as part of the `tf.data` checkpoint restore process,
which might load an iterator whose state includes functions that
aren't present in the original graph. (This is currently achieved by
creating an isolated `FunctionLibraryRuntime` for each function-using
`Dataset`, but that is inefficient and prevents using features of the
main runtime, such as cross-device function calls.)

PiperOrigin-RevId: 181352217