Provide some default behavior on Slack sink Kamelet

[christophd]

The Slack sink Kamelet uses the Webhook API in Slack and this assumes some special Json format in the message body (e.g. {"text": "Hey, you've won a prize"}).

At the moment we expect the user of the Kamelet to provide this Slack specific Json body. I think we can add some default behavior into the Kamelet to automatically wrap the message body content (e.g. when plaintext message body is given) to auto generate the Slack Json format.

This would ease the use of this Kamelet a lot.

[christophd]

sorry my bad. the underlying Camel component takes care of everything. so we can already provide the plain text to the Kamelet that gets posted to the Slack channel.