Generate binding constructs for YANG extensions

We are generating constructs for YANG feature statements for the purposes of specifying run-time support.

A similar capability is needed, which is the requirement for YANG parser to interpret specific YANG extensions, so their effects are not ignored at runtime.

Define a YangExtension yang.binding base class, similar to YangFeature introduced for features in MDSAL-766, and generate concretizations for each encountered extension. Just as YangFeature, the extension should be exposing a qname() method, which can be used to correlate the construct to a yang-parser-api configuration requirement..