Move sapling_backingstore_get_tree_batch to cxx_bridge

Summary:
cxx.rs provides a more ergonomic and opinionated interop layer between Rust and C++ that we would like to leverage for future API chages.

This moves  sapling_backingstore_get_tree_batch from the existing cbindgen implemenation to the new cxxbridge.

Reviewed By: xavierd

Differential Revision: D51321500