lookahead decoding graph for VOSK style clients

Add lookahead decoding graph for compatibility with VOSK style clients.

These modifications will produce both:

• usual HCLG.fst
• Gr.fst and HCLr.fst for VOSK style client support

Would need a version of Kaldi which supports lookahead composition (versions after March 2020).

Reference:

• https://github.com/kaldi-asr/kaldi/blob/76a97983a1250818cc25aeaa1500fb5256df63b2/egs/librispeech/s5/local/lookahead/run_lookahead.sh