Core XSD: move 'charset' attribute declaration to the new 'object-to-string-transformer' complexType

Recent <object-to-string-transformer> improvement introduced new 'charset' attribute.
It works fine, but it is placed in the common 'inputOutputChannelGroup' attributeGroup. It produces confused visual side effect for all other root components.
From other side: <object-to-string-transformer>'s 'charset' isn't available within <chain> now. It's a bug.