Auditing: `ConversionService` support for `CreatedBy`/`LastUpdatedBy`

[gbrehmer]

We have the use case, that createdby is sometimes the username (string) and sometimes the userid (Long), it depends whether data is only for internal use or not.
Currently ConversionService is only supported for date fields. It would be nice, if this could be also possible for auditor fields.
With ConversionService support it would be possible to use a single AuditorAware bean which will be converted to the right target type.
In addition its currently not possible to define two different AuditorAware as an alternative solution.