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)