AGC2 RNN VAD: Spectral features extraction.

This CL defines SpectralFeaturesExtractor which is responsible for
computing the spectral features used as input for the RNN.

Bug: webrtc:9076
Change-Id: I5e1396b89eca9c13bb268e8419a16436a9c3450f
Reviewed-on: https://webrtc-review.googlesource.com/73760
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23206}

modules/audio_processing/agc2/rnn_vad/sequence_buffer.h

@@ -72,6 +72,7 @@ class SequenceBuffer {
   }
 
  private:
+  // TODO(bugs.webrtc.org/9076): Switch to std::vector to decrease stack size.
   std::array<T, S> buffer_;
 };