yang-maven-plugin: allow flexible SchemaSource input

Current yang-maven-plugin implementation requires input schemas to be present in YANG text format. But our model parsing infrastructure allows different inputs, for example YIN.

Design and implement a way for the yang-maven-plugin to ingest alternative formats into the SchemaContext build pipeline. This should allow input formats to be provided at plugin use time, similar to how different code generation plugins are glued it.

The output of processing should result in a YANG text file being generated and packaged in the output. The original input file should also be included.