[Drift] Support optional user-defined class while generating idl

[shangm2]

1. For optional user-defined class, the metadata for it looks like

ThriftType{
protocolType=STRUCT,
javaType=java.util.Optional<com.facebook.drift.idl.generator.TreeNode>, 
valueTypeReference=Resolved reference to 
    ThriftType{
        protocolType=STRUCT, 
        javaType=class com.facebook.drift.idl.generator.TreeNode, 
        structMetadata=com.facebook.drift.idl.generator.TreeNode
    }
}

To print it out while generating idl file, we need to unwrap it.
2. This also include the change from #112 which will be cleared out after it gets merged.