Consolidate use of JSON mappers

[stuebingerb]

Some parts of the code use kotlinx.serialization, some parts use jackson. This is highly confusing and unnecessarily increases complexity.

[stuebingerb]

Target picture should IMHO be kotlinx.serialization. After #247 this could be a bit easier to achieve but one major difference is at least that jackson ObjectNodes are mutable while JsonObjects are not. This might have been one of the reasons for introducing the DeferredJsonBuilder.