Flatten video/audio in StartScreencast command options

[nvborisenko]

This pull request refactors how screencast stream options are handled in the index.bs specification. The main change is the removal of the streamOptions object in favor of passing video and audio parameters directly at the top level of the command parameters. This simplifies the parameter structure and the logic for handling screencast options.

Screencast parameter structure simplification:

• Removed the nested streamOptions object from StartScreencastParameters, and now video and audio options are provided directly as top-level parameters.
• Updated the screencast stream construction steps to reference video and audio directly from command parameters, simplifying the logic for handling width, height, frameRate, and audio options.

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Preview | Diff

[nvborisenko]

audio here might be complex object instead of simple bool (codec, bitrate, etc). But this is different story.