add Private fields to ReadResource

Private data was previously created during Plan, and sent back to the
provider during Apply. This data also needs to be persisteded accross
Read calls, but rather than rely on core for that we can send the data
to the provider during Read to allow for more flexibilty.

internal/tfplugin5/tfplugin5.proto

@@ -219,10 +219,12 @@ message ReadResource {
     message Request {
         string type_name = 1;
         DynamicValue current_state = 2;
+        bytes private = 3;
     }
     message Response {
         DynamicValue new_state = 1;
         repeated Diagnostic diagnostics = 2;
+        bytes private = 3;
     }
 }