String value length

[GCHQDeveloper29]

Currently, string value'd schema are limited to 255 characters in length due to 
OpenJPA defaults.
It would be beneficial to either have this length as adjustable, increased or 
converted to a Lob.
The relevant code is within 
core/persistence-jpa/src/main/java/org/apache/syncope/core/persistence/jpa/entity/AbstractPlainAttrValue.java