[Network] Allow plugins to simulate various network conditions

[abhinavsingh]

Describe the solution you'd like
Add options to simulate various network types (per-client and / or globally) e.g. 2g, 3g, .... Custom profiles can be added by specifying packet drop, latency and other metrics via plugins.

Starter profiles can be found in webpagetest project repo.

[abhinavsingh]

Also see https://github.com/h2non/toxy#poisons for some interesting examples and use-cases.

[sakurai-youhei]

@abhinavsingh Let me advertise :) I wrote one plugin which would simulate a poor network to some extent. My main purpose is not the network simulation but spoiling a user experience, though. Hope it would help those who come here for the same reason in some future.