Do not allow registering shared state type convertor into the shared instance `ConversionService.DEFAULT`

[dstepanov]

Issue description

There are some issues that are causing it https://github.com/micronaut-projects/micronaut-core/pull/7635

Major changes are needed:

• Remove methods that add converters to ConversionService and introduce MutableConversionService
• Allow contributing converters to ConversionService.DEFAULT only by service loader entries
• Register bean context converters to a managed instance of ConversionService that is delegating to ConversionService.DEFAULT just like Micronaut Data is doing https://github.com/micronaut-projects/micronaut-data/blob/a9c1f96ec45f3b4bf8dd250e8342e59e3b99d5b4/data-runtime/src/main/java/io/micronaut/data/runtime/convert/DataConversionServiceImpl.java