gwctl: Introduce a --for flag to filter results related to a specif…

…ied resource. (kubernetes-sigs#3068)

* Remove unnecessary flags exposed by klog and only expose the verbosity (-v)
flag. Get rid of global flag variables like kubeConfigPath.

* Remove duplication in Get and Describe commands.

* Add missing discovery functions discoverGatewaysForGatewayClasses() and
discoverBackendsForHTTPRoutes().

* Add --for flag for filtering resources based on an associated resource.

For example:
* gwctl get backends --for gateway/ns2/my-gateway
* gwctl get gateways --for httproute/ns1/my-httproute
* gwctl get gateways --for gatewayclass/foo-gatewayclass

* fixup! Remove duplication in Get and Describe commands.

* fixup! Add missing discovery functions discoverGatewaysForGatewayClasses() and discoverBackendsForHTTPRoutes().

* fixup! Remove duplication in Get and Describe commands.

* fixup! Add --for flag for filtering resources based on an associated resource.