Define a YangDataName in yang.common

RFC8040 yang-data extension defines a template name. RFC8791 builds on top of it, but there is a subtle difference: 'yang-data' implies a String template name, where as 'structure' implies adressability through absolute-schema-nodeid – which implies schema tree addressability, which implies argument is 'identifier' bound to a namespace.

At any rate, when we generate MD-SAL Java bindings for yang-data, we want to also expose the equivalent of a QName – except here it is a QNameModule and any random string.

Add a YangDataName class, which functions pretty much just like QName, but does not have the localName implications.

Update YangDataStatement to use this common construct.