Migration tool should support customized Externalizer

The current Migration Tool only supports the migration for JDBC cache store from ISPN5.2->ISPN6 without customized externalization.

It is not possible to migrate a cache store where a custom externalizer was used.
It should be possible to set the Externalizer with the properties and add it to the classpath.

As it is no difference whether the source and target Externalizer are the same or not, different should be supported as well to migrate the store to another Externalizer.