Allow for multiple provider configs, and for merging multiple configs. (

#1003)

This is mostly to enable overlaying a second config on top of the first
through runtime options.

This now works:
```
      "session_options": {
        "provider_options": [
          {
            "cuda": {}
          },
          {
            "rocm": {}
          }
        ]
      },
```

src/config.cpp

@@ -33,8 +33,13 @@ struct ProviderOptionsObject_Element : JSON::Element {
   explicit ProviderOptionsObject_Element(std::vector<Config::ProviderOptions>& v) : v_{v} {}
 
   JSON::Element& OnObject(std::string_view name) override {
-    if (options_element_)
-      throw std::runtime_error("Each object in the provider_options array can only have one member (named value)");
+    for (auto& v : v_) {
+      if (v.name == name) {
+        options_element_ = std::make_unique<ProviderOptions_Element>(v);
+        return *options_element_;
+      }
+    }
+
     auto& options = v_.emplace_back();
     options.name = name;
     options_element_ = std::make_unique<ProviderOptions_Element>(options);