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.
In addition to exposing compilation over the API cleaned up the structures that would be exchanged to be cleaner and easier to support and understand.
Also removed ability to configure the envoy OverprovisioningFactor.
