Support for running commands and subscribing to events for multiple user contexts

[whimboo]

We should discuss the possibility of enhancing WebDriver BiDi to support running commands and subscribing to events for a specified list of user contexts. This would provide more granular control over how commands are issued and events are captured in multi-user context scenarios.

Here some examples:

1. Use the browsingContext.setViewport command to set the viewport for all the tabs from user context 1 at the same time. Same could be imagined for a future user agent emulation.
2. Subscribe to events like browsingContext.contextCreated only for tabs of a given user context.