New class SchemaHolder

A simple AsJsonValueHolder for a SchemaTree.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Change inputs/outputs of RefResolverProcessor and SyntaxProcessor

Prior to this change, they were accepting ValidationContexts as inputs.

They now take SchemaHolders.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>