Let user configure `Access-Control-Allow-Origin` header

[relu91]

As discussed in our , in certain use cases users might what to restrict usage of WebThing affordances to a certain Origin. Currently, the Access-Control-Allow-Origin header is hardcoded in our http-server. A nice to have would be to configure that value using an option parameter with the default value being *.

[jona42-ui]

@relu91 are you able to assign me this issue?