This diff moves the Rust notifications code into the `notifications` submodule as a part of the transition to using a single Rust library linked to the C++ codebase.
This is a part of the stack.
The full context is available in the Linear task.
Related Linear task: [[ https://linear.app/comm/issue/ENG-1895/change-to-use-a-single-rust-code-library-to-c-code-instead-of-a-few | ENG-1895 ]]