Configuring Exposed Checks for non-sidecar style proxies should not be allowed nor cause proxycfg errors

If you register a non-sidecar style proxy (the `DestinationServiceID` field is unset) then the proxycfg package will still attempt to start watches for exposed checks. These produce spurious errors in the logs.

Additionally we should probably add a validation where we validate the rest of the expose config to ensure that exposed checks being enabled are only allowed for a sidecar style proxy.