allow setting runtime handles (alternative)

[fs-eire]

This PR is an alternative way of #997 to implement allowing users to set runtime handles.

The basic idea is to create a JSON string representing the Config class from the RuntimeSettings, and the Config class will merge them.

Currently this PR is not working because of the following:

• The existing JSON parser does not support multiple entries in provider_options
  The following JSON will fail to parse.

  {
    "model": {
        "decoder": {
            "session_options": {
                "provider_options": [
                    {
                        "webgpu": { }
                    },
                    {
                        "dml": {}
                    }
                ]
            },
        },
     },
  }
  

• When trying to specify JSON overlay, two "webgpu" items does not merge.
  genai_config.json:

  {
    "model": {
        "decoder": {
            "session_options": {
                "provider_options": [
                    {
                        "webgpu": { "abc": "123" }
                    }
                ]
            },
        },
     },
  }
  

  generated overlay config:

  {
    "model": {
      "decoder": {
        "session_options": {
          "provider_options": [
            {
              "webgpu": {
                "dawnProcTable": "12345678"
              }
            }
          ]
        }
      }
    }
  }
  

  Result:

  

This PR depends on a code change to the Config class to support the expected parsing behaviors.

[fs-eire]

After merged to latest main (takes #1003), it should work now. Need test whether it actually works.

[guschmue]

tested, works well.