[SWF Dev UI] Add support to start workflows using REST API and cloud events

Cloud Event form page

It can be accessed from the Workflow List to help you interacting with active instances or from the Definitions page to start new instances. This screen should be redesigned to allow user fill the needed data to trigger the cloud events via HTTP (no kafka yet).

The main fields are:

• Endpoint: method & url to post the cloud event to
• Event Type: Type of cloud event to be triggered
• Event Source: header specifying the event source
• Instance Id: fill it with the id of an active SW instance (only visible when accessing from the SW List)
• Business Key: fill it with a business key to start a new SW instance (only visible when accessing from the SW Definitions List)
• Custom Headers: Allows adding custom headers (key-value pairs) to the cloud event
• Event Data: fill it with the JSON data you want to send in the cloud event

Workflow Form page

This screen should be used when starting an instance of a SW via REST, it can be accessed from the Definitions page by clicking on the play button next to any SW in the table.
This screen should be reworked to enable displaying a custom form (if the SW has a `dataInputSchema` defined) or a CodeEditor to help the user typing the payload to start the WF instance.

Previously this screen was trying to post a CloudEvent if no form was defined, which was wrong since not all our examples accept CloudEvents as start.

Acceptance criteria

The feature is implemented as per points above.
The bug mentioned in comment below is fixed.

Testing

Unit tests are added with implementation pull-request.
Manual tests for happy path scenarios are performed during PR review.
Automated tests are optional at the current stage.