[Feature]: Add "On behalf of" option

[dsbaars]

Question or Feature?

Great library!
At this moment all tickets, objects etc. are "created by" the user that created the token.
We want "created by" to be set to a certain user. This is possible by adding a "X-On-Behalf-Of" header to the request, see https://docs.zammad.org/en/latest/api/intro.html#actions-on-behalf-of-other-users

It would be nice if we could set this header similar to how the official Zammad API client allows to do this.

[StanBarrows]

Hey @dsbaars

At the moment we do not plan to implement this but I would accept a PR.

Best regards
Sebastian