Support custom path to the `rustfmt` executable.

This CL avoids hardcoding ".../bin/rustfmt" path in the rustfmt function
in token_stream_printer.rs. This is replaced with an argument of a new
cmdline parameter: --rustfmt_exe_path.

PiperOrigin-RevId: 451159464

rs_bindings_from_cc/src_code_gen.h

diff --git a/rs_bindings_from_cc/src_code_gen.h b/rs_bindings_from_cc/src_code_gen.h
index fc942c7..d59e2e9 100644
--- a/rs_bindings_from_cc/src_code_gen.h
+++ b/rs_bindings_from_cc/src_code_gen.h
@@ -22,6 +22,7 @@
 
 // Generates bindings from the given `IR`.
 Bindings GenerateBindings(const IR& ir, absl::string_view crubit_support_path,
+                          absl::string_view rustfmt_exe_path,
                           absl::string_view rustfmt_config_path);
 
 }  // namespace crubit