Remove inline namespace imports

[christianhelle]

This resolves #68

The fixes the issue when using system as a property name in the OpenAPI specifications. The problem was that it was conflicting with the [System.Text.Json.Serialization] attributes in the generated contracts

{
  "swagger": "2.0",
  "info": {
    "title": "XX",
    "version": "0.0.0"
  },
  "host": "x.io",
  "basePath": "/",
  "schemes": [
    "https"
  ],
  "definitions": {
    "Dummy": {
      "type": "object",
      "properties": {
        "system": {
          "type": "string",
          "description": "XX"
        }
      },
      "description": "XXX",
      "example": {
        "system": "Dummy"
      }
    }
  }
}

This was resolved by globally importing the System.Text.Json.Serialization namespace