Add ServerMaintenance API type

[afritzler]

Proposed Changes

Fixes #76

[stefanhipfel]

would it make sense to add a time slot property when to perform maintenances, or should this be done outside of the ServerMaintenance operation?
e.g. I want to allow maintenances only during weekends or at a specific timestamp:
Customer gets notified (Email or dashboard) that there are maintenances pending and needs to define a time when those should be applied.

Currently this could be done setting the OwnerApproval label at the right moment, but this is a manual step or needs another service which automates that step. I think having that implemented in the ServerMaintenance operator makes for a great feature, which is something we need at ccloud.