Ability to pass a FactoryInstance to WaveformAudioBufferSink which will create custom WaveformBar instances

[SathyaTadepalli]

If a consumer app wants to override the behavior of WaveformBar it is not possible, because the creation is happening internally in WaveformAudioBufferSink.
One option is to have a override constructor for WaveformAudioBufferSink, which takes an Factory that will be responsible for creating WaveformBar.
Consumer app can pass this factory instance and create custom WaveformBar instances.
The current constructor which is present will be still available and the current WaveformBar will be created.
There will be no impact in current behavior

Added unit test for the custom factory.

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[microkatz]

@SathyaTadepalli

If you could please sign the CLA agreement first then that would be great! Thanks!

[icbaker]

Closing because the CLA hasn't been signed in 3 weeks. Please re-open or file a new PR if you are able to sign the CLA.