grpc-js-xds: Shutdown the xDS client used by the resolver when the channel shuts down

[murgatroid99]

A channel that connects to an xds:/// backend creates an internal client connected to an xDS server. Currently that client just always has an active call and holds the process open forever. This change makes it so that when that channel's close method is called, which calls its resolving load balancer's destroy method, that propagates to destroy the resolver, which then shuts down the xDS client, allowing the process to end.

Technically, this is a breaking change in the experimental namespace by adding an additional method to the Resolver interface that would need to be implemented by an implementing class. However, the real point of the experimental version compatibility guarantees is to avoid breaking grpc-js-xds, and since that's not out yet I don't consider it to be a real issue.