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](https://github.com/istio/istio/blob/99f2713f2808e94de72587fed7bf0931bdb5ad02/pkg/bootstrap/config.go#L332-L335) 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](https://github.com/getyourguide/proxy/pull/2/files#) is patching the tool overriding this runtime for [my use case](https://github.com/getyourguide/istio-config-validator/tree/c4d85fec29ee0500fac60d1de99871c1afb749af/hack/istio-router-check).

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?