Ensure nothing in the troubleshoot go module depends on consul's top level module. This is so we can import troubleshoot into consul-k8s and not import all of consul.
* turns troubleshoot into a go module [authored by @curtbushko]
* gets the envoy protos into the troubleshoot module [authored by @curtbushko]
* adds a new go module `envoyextensions` which has xdscommon and extensioncommon folders that both the xds package and the troubleshoot package can import
* adds testing and linting for the new go modules
* moves the unit tests in `troubleshoot/validateupstream` that depend on proxycfg/xds into the xds package, with a comment describing why those tests cannot be in the troubleshoot package
* fixes all the imports everywhere as a result of these changes
Co-authored-by: Curt Bushko <cbushko@gmail.com>
