* Adds 'limits' field to the upstream configuration of a connect proxy
This allows a user to configure the envoy connect proxy with
'max_connections', 'max_queued_requests', and 'max_concurrent_requests'. These
values are defined in the local proxy on a per-service instance basis
and should thus NOT be thought of as a global-level or even service-level value.
Compiling this will set an optional SNI field on each DiscoveryTarget.
When set this value should be used for TLS connections to the instances
of the target. If not set the default should be used.
Setting ExternalSNI will disable mesh gateway use for that target. It also
disables several service-resolver features that do not make sense for an
external service.