Fixes: #5396
This PR adds a proxy configuration stanza called expose. These flags register
listeners in Connect sidecar proxies to allow requests to specific HTTP paths from outside of the node. This allows services to protect themselves by only
listening on the loopback interface, while still accepting traffic from non
Connect-enabled services.
Under expose there is a boolean checks flag that would automatically expose all
registered HTTP and gRPC check paths.
This stanza also accepts a paths list to expose individual paths. The primary
use case for this functionality would be to expose paths for third parties like
Prometheus or the kubelet.
Listeners for requests to exposed paths are be configured dynamically at run
time. Any time a proxy, or check can be registered, a listener can also be
created.
In this initial implementation requests to these paths are not
authenticated/encrypted.
