Refactor[MQBSTAT]: fix unnamed namespace forward declaration warning

[678098]

To fix the warning introduced before:

2024-07-23T07:25:38.9243161Z /home/runner/work/blazingmq/blazingmq/src/groups/mqb/mqbstat/mqbstat_jsonprinter.h:51:7: warning: ‘BloombergLP::mqbstat::JsonPrinter’ has a field ‘BloombergLP::mqbstat::JsonPrinter::d_impl_mp’ whose type uses the anonymous namespace [-Wsubobject-linkage]
2024-07-23T07:25:38.9245536Z    51 | class JsonPrinter {

• Remove forward declaration from the unnamed namespace in the header
• Make JsonPrinterImpl a private forward declared type to JsonPrinter
• Extract static methods from JsonPrinterImpl to ConversionUtils placed in the unnamed namespace
• Reduce the build log from 8000 lines to 7636 by getting rid of this warning