Add generateFieldErrorWithKnownKeys function

[bilgeyucel]

Add generateFieldErrorWithKnownKeys function to use instead of using generateErrorMessage with custom error

const exampleFour = {
  type: "ValidationError",
  detail: {
    password: ["Password is too short", "Please use only letters and numbers"],
    message: {
      body: ["Message body is missing"],
      attachment: ["Attachment is missing"]
    }
  },
  fallback_message: "This is a random fallback message"
};

generateFieldErrorWithKnownKeys("message", knownErrorKeys:["body"]) // ["Attachment is missing"]