Add an immediate YangTextSchemaSource

We have downstreams who are caching YangTextSchemaSource content as a constant byte[].

Add ImmediateYangTextSchemaSource, which holds a byte[], possibly with a symbolic name, which does all the (mediately) heavy lifting.

While we are at it, the requirement to override addToStringAttributes() in both Yang and Yin text schema sources seems excessive: we provide the identifier by default. Make this method non-abstract, so that users are not required to provide this implementation.

This necessitates updating the calling convention, which is a bit of pain. To make up some bonus points, configure ToStringHelper to ignore null values.