[native] introduce RustCallback

Summary:
we introduce a generic C++ callback function to be called from Rust

note that this function is only used when the js return type is Promise<string>

we'll have to add similar functions to RustCallback for other return types
luckily we really only ever return strings from rust...

Test Plan: this gets tested in the next diff in stack

Reviewers: ashoat, jon

Reviewed By: ashoat, jon

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7214