Add plugin extension point to add Spring Converters to the headless server

Add new plugin extension point to add one or more Spring Converter , to convert among other things the GraphQL input types which is needed when adding custom input types in plugins.

(CMS-20711)

Automatic conversion of input types to Java objects for adapters

The graphQlConversionService is a standard Spring org.springframework.core.convert.ConversionService enhanced by all instances of org.springframework.core.convert.converter.Converter created as regular Spring Beans.

Using this ConversionService together with the Spring Expression Language, adapters in the content-schema now may use typed parameters instead of a composition of collection objects deserialized when using GraphQL input types. If a suitable Converter is available, the ConversionService will invoke the conversion transparently.

(CMS-20327)

Support for viewtype specific queries

The Headless-Server now supports viewtype specific queries for CMCollection and PageGridPlacement. See documentation for details.

(CMS-19709)