feature: Allow selection of different Orchestrator model

[taicchoumsft]

Description

Provides the backend to allow the user to change the Orchestrator model from the defaults. A UI will be provided in a future PR.

The user can change the model by adding the followint to appsettings.json:

"orchestrator": {
    "model": {
      "en_intent": "pretrained.20200924.microsoft.dte.00.03.en.onnx",
      "multilingual_intent": "pretrained.20200924.microsoft.dte.00.03.multilingual.onnx"
    }
  }

If this section is blank or unreadable, we simply use the Orchestrator defaults. If the model is not in the supported Orchestrator list, we throw an error message.

A future PR will allow the user to specify the model download location as well.

Task Item

closes #6400

[coveralls]

Coverage decreased (-0.02%) to 51.672% when pulling 11b00db on tachou/orchSelectModelV2 into 0509422 on main.