Fix ClusterLoadAssignment timeouts dropping endpoints.

When a large number of upstreams are configured on a single envoy
proxy, there was a chance that it would timeout when waiting for
ClusterLoadAssignments. While this doesn't always immediately cause
issues, consul-dataplane instances appear to consistently drop
endpoints from their configurations after an xDS connection is
re-established (the server dies, random disconnect, etc).

This commit adds an `xds_fetch_timeout_ms` config to service registrations
so that users can set the value higher for large instances that have
many upstreams. The timeout can be disabled by setting a value of `0`.

This configuration was introduced to reduce the risk of causing a
breaking change for users if there is ever a scenario where endpoints
would never be received. Rather than just always blocking indefinitely
or for a significantly longer period of time, this config will affect
only the service instance associated with it.