[enhancement] Added support for Incoming Webhook

[m5kr1pka]

Summary

Closes #11 Advanced messages fails issue, because slack only accepts not flattened parameters and payload doesn't reach workflow builder afterwards.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[li-olivierfostier]

Pleaaaase Hurry up ! :)

[akasakashota]

Did you forget to build it? I'm looking forward to this PR being merged. Thanks.

[gokaygurcan]

Any update on this?

/cc @stevengill

[seratch]

LGTM but as I mentioned at #11 (comment), I would like to wait for @stevengill's review for this

[seratch]

Oops, I missed that the current implementation support only Workflow Builder webhook use cases. To support both webhooks for posting a message nad webhooks for triggering a workflow, perhaps, we need to:

• have a new option to determine the webhook type
• dispatch the logic accordingly (=the workflow webhook pattern should be supported too)

[stevengill]

Agree with @seratch. I left a comment on the open issue. We don't want to remove flatten from the library but want to introduce a fork for incoming_webhook support.

[m5kr1pka]

cc'ing: @seratch @stevengill

• Implemented a webhook type via env parameter: SLACK_WEBHOOK_TYPE: BUILDER or INCOMING, by default: BUILDER (existing);
• Updated documentation with Incoming webhook;
• added incoming webhook example to example-workflows;
• ran build script and committed unfortunately;
• added @vercel/ncc to devDependencies;

(phew)

P. S.
Would be great to implement tests in this action, but major release would be required

[seratch]

A few comments. Please wait for @stevengill's review before starting to modify this PR

[m5kr1pka]

@seratch changes look good to me, thanks for the review, waiting for confirmation.

[stevengill]

LGTM