FormSG Zapier: Add endpoint to retrieve sample submission data

[wanlingt]

To improve the experience of setting up the Zapier integration, we should provide test submission data through an endpoint. Users will no longer have to perform a real submission on their form.

When users are setting up a hook-based (aka instant) Trigger, it’s important to have a polling fallback. For example, imagine a Zap that triggers on a new Slack message. If testing relies on the sending of a webhook, the test won’t complete without the user sending an actual message in a Slack channel, which is disruptive.

Instead, during testing, the Perform List (performList) operation fetches a (real) recent message using the provided URL for polling and uses it as the test result. The polling URL is only used for tests.

More context here