Feature request: validation keys for ChannelOptions, ClientOptions, and plugins

[SimonWoolf]

We just discovered that some of the Delta docs wrongly specified channels.get(channelName, {delta: 'vcdiff'}) instead of channels.get(channelName, {params: {delta: 'vcdiff')}}. I'm guessing Tom in writing the docs tried the former, but it seemes do work fine and didn't error, so assumed it was correct. Would be nice if ably-js errored if you pass a nonexistent key into channel or client options, to catch those mistakes earlier.

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