[.NET] Multiline Input.Text elements should stretch when height is set to stretch

.NET spin off of #4097

Platform

• .NET

Details

For multiline Input.Text elements with the height set to stretch, the control should stretch to take up the available space. Sample card below:

{
      "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
      "type": "AdaptiveCard",
      "version": "1.0",
      "body": [
        {
          "type": "TextBlock",
          "size": "Medium",
          "weight": "Bolder",
          "text": "Custom JSON Design Input",
          "horizontalAlignment": "Center"
        },
        {
          "type": "TextBlock",
          "text": "In addition to showing your our pre-built samples, I can submit your custom design for you.  This helps to see how it will render or to see what type of payload your app will need to process if a user hits an Action.Submit button.",
          "wrap": true
        },
        {
          "type": "TextBlock",
          "text": "One way to get design JSON to paste here is to use the \"Copy Card JSON\" button from the [Buttons and Cards Designer](https://developer.webex.com/buttons-and-cards-designer) "
        },
        {
          "type": "Container",
          "items": [
            {
              "type": "Input.Text",
              "placeholder": "Paste card JSON here",
              "id": "cardJson",
              "isMultiline": true,
              "height": "stretch",
              "maxLength": 8000
            }
          ],
          "minHeight": "266px"
        }
      ],
      "actions": [
        {
          "type": "Action.Submit",
          "title": "Display My Card",
          "data": {
            "cardType": "customJsonInput",
            "nextAction": "submitJson"
          }
        },
        {
          "type": "Action.Submit",
          "title": "Go Back To The Built In Samples",
          "data": {
            "cardType": "customJsonInput",
            "nextAction": "samplePicker"
          }
        }
      ]
    }

Start with the .NET implementation of the multiline Input.Text element and reproduce the behavior using the supplied Adaptive Card JSON, especially the stretch height and minHeight settings. Done means the control expands to fill the available container space when multiline and height is set to stretch.