[C++] IrDecoder should expose the schema id and schema version

[aschw]

https://github.com/aeron-io/simple-binary-encoding/blob/048e3876befa8eb834f59eb1d18e615f29ebfda4/sbe-tool/src/main/cpp/otf/IrDecoder.h#L219

m_id currently has no users. It should be exposed with a public getter. frame.schemaVersion() should also be extracted in decodeIr() and made visible with a public method.