Add a proxy server support

[zhiguang-chen]

A http to http or https to http proxy.

[zhiguang-chen]

It is part of the mock server, just set a foward:

    {
      "name": "AWS Sam Local",
      "method": "GET",
      "path": "aws",
      "cases": [
        {
          "forward": "https://localhost:3043"
        }
      ]
    }