Add support of full-page screenshot configuration

[YevgeniyShunevych]

{
  "takeScreenshotOnNUnitErrorKind": "default", // Supports: "default", "viewport" and "fullPage".

  "screenshots": { // Configures screenshots functionality.
    "strategy": {
      "type": "webDriverViewport", // Supports: "webDriverViewport", "webDriverFullPage", "cdpFullPage", "fullPageOrViewport",
                                   // and name of custom type implementing "Atata.IScreenshotTakeStrategy".
      "{{strategyValueName}}": "value" // Any property or constructor parameter of strategy.
    }
  }
}