@Tomasz KocielaWelcome to the community!
With the information you have provided, and my understanding is,
So the simple answer to this problem is NO, you can not host/setup any web service in Jira Cloud.
You can always link your existing web service (hosted at some other place) with Jira using rest api and plugin interface provided by Jira Cloud.
PS - If my understanding is not correct please elaborate on your use case, and come back. I will be happy to help.
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