enhance router_check_tool to allow going through listeners as well

Currently, router_check_tool takes in a route as input. This makes the tool somewhat whitebox - the user first needs to figure out which route they care about, then they can write a test for it. Ideally, its completely black box - the user gives the full Envoy config (really just Listeners and Routes) and a set of queries, and checks the result.

This is useful as well to test complex listeners with many different filter chain matches.

Alternatives considered:

• Implement the logic for determining a route from the request ourselves. This would involve re-implementing the filter chain/listener matching logic which is likely to be incorrect and fall out of sync
• Use the tool as is. This doesn't mean the requirement of black box testing, relies test writers to know Envoy implementation details
• Send real traffic against a real envoy instead. This is slower and more complex