Fixes https://github.com/spring-projects/spring-integration/issues/8757
* Add ctor `ObjectStringMessageConverter(Charset)`
* create tests for `ObjectStringMessageConverter`
* fix codestyle issues
* revise JavaDoc
* `@NonNullApi` for `org.springframework.integration.support.converter`
* revise `SimpleMessageConverter`
* `inboundMessageMapper` and `outboundMessageMapper` are initialized to their default implementations
Bboth are set in non-default ctors with final setters
* remove `@Nullable` from `private BeanFactory beanFactory`
* Handle empty `String... headerNames`
* Some code clean up
29186e2b46