rpc : add support for multiple devices (#16276)

* rpc : add support for multiple devices

Allow rpc-server to expose multiple devices from a single endpoint.
Change RPC protocol to include device identifier where needed.

closes: #15210

* fixes

* use ggml_backend_reg_t

* address review comments

* fix llama-bench backend report

* address review comments, change device naming

* fix cmd order

ggml/src/ggml-backend-impl.h

@@ -209,9 +209,6 @@ extern "C" {
         void * context;
     };
 
-    // Internal backend registry API
-    GGML_API void ggml_backend_register(ggml_backend_reg_t reg);
-
     // Add backend dynamic loading support to the backend
 
     // Initialize the backend