[CvTooltip] Add ability to add custom classes to the content container

[TimonLukas]

Is your feature request related to a problem? Please describe. I'm trying to change the styling of some CvTooltip content containers, but not all. Currently I don't see a good way to do this as all CvTooltip content containers are appended to the body, which means I can't use hierarchical CSS to target them. At the same time I don't see a way to add custom attributes like a class since the content is added through a slot.

Describe the solution you'd like I think adding an optional prop contentClass: string could solve this problem elegantly.

Describe alternatives you've considered I can't come up with any good ones.