Deprecate POST action

[zubairov]

WebHook Action POST should be depricated and phased out. Please use REST API Client instead.

[zubairov]

Related issue EIOT-577

[jhorbulyk]

So there are some advantages offered by WebHook POST that aren't yet provided by the REST API Client:

• Auto-setting of Content-Type and Accept HTTP Headers
• Setting the body of the call to the incoming message body
• I can't remember if this component supports sending attachments

[khanzadyan]

My suggestion is to deprecate it. The effort of trying to fix this is probably too much. I've checked in our DB there are 30 active flows which use WebHook post. So we need to add a deprecation flag and leave it like that for some time.

[jhorbulyk]

Item is deprecated. Next step may be to fully remove the action and unrelated dependencies.