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

[RyanUnderhill]

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": {}
          }
        ]
      },

[fs-eire]

looks like the linux CUDA pipeline has flaky tests.