[step-extension] Slack Source

[Delawen]

Create an extension for https://camel.apache.org/camel-kamelets/latest/slack-source.html

Instructions to create a new extension in https://kaoto.io/docs/add-custom-view/

This extension must allow the user to set up all the configuration options of the kamelet and:

• Validate the token property has the proper format
• Add a test button to check the configuration

[joshiraez]

I'll take this one

[kahboom]

@Delawen should we move these step extension issues to the step-extension-repository?