SchemaBuilder: Support making validation of default values configurable

The SchemaBuilder's createFields methods pass a value of 'true' to the validateDefault option within the Field constructor.

While this is very useful to have, it results in not being able to seamlessly migrate from 1.8 to >= 1.9 if an application/registry already has schemata with invalid defaults. 

Given that Schema.Parser allows us to configure this, can we support the same idea within SchemaBuilder?