FEATURE: Let's add an HTTP header so we know where the user is running a function

[dzwillia]

Let's add an HTTP header to our API calls so we know where they're being issued from:

headers: {
  "X-Requested-From": "googlesheets"
}

Obviously, the header key value can whatever you deem best, but I think the practice will be good for us knowing where calls are coming from. If we do this, I can update the admin area with richer information other than just "Api Request", etc.

[dzwillia]

Done.

(We used "X-Flexio-Source")