[Audio] Add AveragePooling1D layer

[zaleslaw]

We are missing some layers to support the export of models from Keras fully. One of them is the AveragePooling1D layer.

Add a layer class, write documentation for it, write a test for it, try, if possible, create a small trainable network with it (in your own GitHub) and attach a link here in the comments.

The layer should be placed here

As a reference implementation, the AvgPool2D layer could be used, but feel free to improve it!

If some refactoring to pooling layers could be applied, please, try to do it!

Also, support for export and import of layer in JSON format should be added (see ModelLoader.kt and ModelSaver.kt)

A detailed description of the layer can be found here

P.S. There are no-ops for tf.nn.avgPool1d in Java API, so you could try to implement it from scratch via available ops or use tf.nn.avgPool like in TensorFlowJS with reshape in tfjs-core/src/ops/avg_pool.ts

[mkaze]

Since this is very similar to MaxPool1D layer, I'll take it and work on it.