Support NCHAR / NVARCHAR for JDBC tables

[kupferk]

Many relational databases now offer different string types, either CHAR(n)/VARCHAR(n) or NCHAR(n)/NVARCHAR(n). The later ones support unicode while the first ones are more resitricted. Flowman can manage schemas in relational databases, but currently always uses NCHAR/NVARCHAR (if available). Since these column types will consume more storage capacity, it is desirable to be able to explicitly specify either NCHAR or CHAR