Option to serialize nonStructuredArguments using toString

[schmidti159]

As described in #304 all arguments are serialized using jackson when includeNonStructuredArguments=true is set.

This might expose sensitive data through logging as not all developers might be aware that the complete object is serialized.

It would be great to have a separate configuration like serializeNonStructuredArguments (default true).
If it is set to false the arguments will not be serialized with jackson but toString will be used.

This enables the opportunity to enable includeNonStructuredArguments=true for legacy code bases where it is not feasible to analyze all existing log messages.

Does it make sense to you to add this to this lib? If yes, I could provide a PR.

[philsttr]

Makes sense. Contributions welcome.

I'd prefer the new flag to be named writeNonStructuredArgumentsAsString, default false.