OZG-5776 xta-adapter: Reuse XtaMessageMapper for fim

...c/main/java/de/ozgcloud/eingang/fim/FimMessageMapper.java

...in/java/de/ozgcloud/eingang/fim/FimXtaMessageHandler.java

...in/java/de/ozgcloud/eingang/xdomea/XdomeaMessageData.java

...a/de/ozgcloud/eingang/xdomea/XdomeaXtaMessageHandler.java

.../ozgcloud/eingang/xta/RepresentationsAttachmentsPair.java

...c/main/java/de/ozgcloud/eingang/xta/XtaMessageMapper.java

...ava/de/ozgcloud/eingang/fim/FimXtaMessageHandlerTest.java

.../ozgcloud/eingang/xdomea/XdomeaXtaMessageHandlerTest.java

...ingang/xta/RepresentationsAttachmentsPairTestFactory.java

...st/java/de/ozgcloud/eingang/xta/XtaMessageMapperTest.java