Backport of all changes related to servicedesk

[Fank]

What type of PR is this?

• feature
• api-change

What this PR does / why we need it:

Its a backport from all related changes in servicedesk from https://github.com/mcl-de/go-jira/tree/fk_internal

Which issue(s) this PR fixes:

Related to #459, #458, #559, #560 & #561

Special notes for your reviewer:

Servicedesk API is a different API module compared to the Jira REST API, so i changed the naming of some of the files with a "servicedesk_" prefix and moved the models into a subfolder called servicedesk.
Not sure about the models subfolder yet, if the names are always different and if similar in name and properties this subfolder can be merged into the models folder.
But otherwise its way harder to split them again if we notice a difference.

Additional documentation e.g., usage docs, etc.:

[Fank]

I would check tests and other stuff which is missing when doing #559 (comment)

[andygrunwald]

Thanks for this Pull Request. It is still on my list to review. It might take a few days, thanks for understanding.