Add optional framerate or max_framerate to VideoDecoderConfig

[taste1981]

Codecs typically have level constraint related to maximum luma samples/ macroblocks per-second. As such, for isConfigSupported(VideoDecoderConfig config), if level is part of the codec string in config, the maximum possible framerate should be specified in config, which the agent will use in combination of resolution, and the level info in codec string, to decide if the decoder could safely claim support of current codec/profile/level.

The framerate is currently missing from the VideoDecoderConfig. It should be added as optional to facilitate isConfigSupported(). It is encouraged for application to specify the framerate on decoder creation as well, to avoid the agent decoding at an extremely high FPS that exhausts resources.