router check tool: configure runtime flags

Title: Load runtime flags in router check tool

Description:

We should be able to configure the router check tool with runtime flags as it can influence routing decision/validation.

Use case:

My goal is to make the tool useful with Istio configuration, Istio uses some runtime flags which if not set limits the use of router check tool quite a bit, especially as we have large regex matching routes and the default value of re2.max_program_size.error_level is 100.

My ugly workaround is patching the tool overriding this runtime for my use case.

I see other issues (https://github.com/envoyproxy/envoy/issues/12393, https://github.com/envoyproxy/envoy/issues/13065) that would also benefit from reading the whole envoy configuration instead of just http routes. Would this be the way to go?