document auth property in webhooks section of processing chain

[ninsbl]

This PR is an attempt to document the auth property in the webhooks section of a processing chain. Currently, this is not visible in swagger.json / ReDoc. I have not tested it and I am not sure this is the right way to add to the documentaion...

It would be good it auth would support base64 encoded strings (instead of username and password in plain text), or even better, if actinia had secrets management (like e.g. in github actions, see also: #398).