WIP: Make strings valid UTF-8 before JSON-encoding them

[Al2Klimov]

fixes #4703

[Crunsher]

@Al2Klimov Any update on this? As @dnsmichi mentioned Utility::ValidateUTF8() is too expensive to use here.

Maybe there is a better way in the mindset of "don't ask for permission, ask for forgiveness" by forcing it to UTF8 and living with a possibly garbled output

[Al2Klimov]

@Crunsher Maybe there is another developer w/ a mindset of "I have time to do this stuff carefully". Right now I am focussed on a bunch of non-Icinga2 things.

Btw I still think that my fix is "the right way" to do this as JSON has to be UTF8-encoded.

[dnsmichi]

@Crunsher I would create a reminder/discussion ticket if not already existing and close this PR. This won't be merged anyways.