Extract name-resolution logic from ManagedChannelImpl for re-use

[zhangkun83]

First mentioned in another comment, and then discussed with @ejona86 and @dapengzhang0, it was concluded that we need to extract the following logic:

1. Parse the target string and convert it into a URI
2. Manage NameResolver retries
3. Wire up NameResolver and LoadBalancer

out of ManagedChannelImpl, and make it usable in the following cases:

1. CHANNEL: ManagedChannelImpl
2. OOB: LoadBalancer.Helper#createResolvingOobChannel (blocking Implement LoadBalancer.Helper#createResolvingOobChannel #5495)
3. CHILD_POLICY: XdsLoadBalancer "custom mode": XdsLoadBalancer receives the target name for each locality, and needs to resolve the target name and start a child LoadBlanacer for it.

[zhangkun83]

For CHILD_POLICY case, our preliminary API design looks like:

// LbConfig class name is a placeholder for whatever type we choose for passing 
// parsed LoadBalancingConfig

abstract class LoadBalancer.Helper {
  RequestDirector createRequestDirector(String target, LbConfig initialConfig, Helper helper);
}

// A RequestDirector is a pair of NameResolver and LoadBalancer
abstract class RequestDirector {
  abstract void handleSubchannelState(Subchannel subchannel, ConnectivityStateInfo newState);
  abstract void shutdown();
  abstract void handleLbConfigChange(LbConfig lbConfig);
}

[zhangkun83]

With the changes made in #5722, we can get rid of handleSubchannelState() from the utility API. By introducing NameResolver.Args and making it accessible from LoadBalancer.Helper, LoadBalancer can now create NameResolvers when they need, thus we no longer need to have LoadBalancer.Helper to instantiate the utility, which is now under a new name:

final class ResolverBalancerPair {
  // Resolves a target and use the given LbConfig to create the LB.
  static ResolverBalancerPair newPair(
      NameResolverRegistry, String target, ResolutionResultFilter, NameResolver.Args, Helper);

  // Shuts down both NR and LB
  void shutdown();

  interface ResolutionResultFilter {
    LoadBalancer.ResolvedAddresses filter(NameResolver.ResolutionResult result);
  }
}

ResolutionResultFilter is called on every ResolutionResult from NameResolver. It will have different behaviors in the three cases:

• In ChannelImpl, this will implement service config handling, extract LB config, parse it, and include it in the ResolvedAddresses.
• In OOB, will only take the addresses and put it in ResolvedAddresses. The service config will be discarded.
• In CHILD_LB, will take the addresses but combine them with the current child LB config from the parent, which is already parsed. The service config from ResolutionResult will be discarded.