[TRIGGER] Slack 'workflow step' to send form data to Pipedream

# Overview

_Support for Slack workflow steps._

Slack provides a quick builder called workflows that enable a user to quickly pull together a way of submitting forms through a Slack workflow action. In order to pass this data through to an external service, such as Pipedream, there needs to be a 'workflow step' that is part of the Pipedream Slack app.

# References

To see more generic information about using workflows (for end users) [Slack's documentation is here](https://slack.com/intl/en-gb/help/articles/360035692513-Guide-to-Workflow-Builder).

For developers, they refer to [this place in the API documentation](https://github.com/PipedreamHQ/pipedream/issues), for details on building workflow steps for a Slack app.

For additional reference, you can [see the Pipedream community slack thread](https://pipedream-users.slack.com/archives/CPTJYRY5A/p1624625345309100) where this was discussed.

# Related

Similarly related, but I think is different and might not cover this use case, would be #1010.