Expose extensions in the ConfigDump endpoint.

*Title*:

Expose extensions in the ConfigDump endpoint.

*Description*:

The set of extensions in the envoy binary are logged at startup, but not otherwise exposed. If these extensions were exposed by the [config_dump](https://www.envoyproxy.io/docs/envoy/latest/api-v2/admin/v2alpha/config_dump.proto) endpoint, then it would be easier for operational tooling (e.g facter) to track what extensions are available. Another use that I have in mind for this is to let Kubernetes ingress controllers inspect an envoy process to ensure that the features it needs are compiled in.