Add option "shareUnitOfWork" to pattern "enrich"

Since CAMEL-5981 in Camel 2.10.4, when an "enrich" is called, its UnitOfWork is "null". This causes our custom producer component to be unable to access the UnitOfWork of the Exchange.
Here is a patch that adds the option "shareUnitOfWork" to the "enrich" pattern classes (Enricher and EnrichDefinition). This implementation was strongly inspired by the implementation of the option in MulticastDefinition and MulticastProcessor.
Patch is against Camel 2.10.7.