grpclb: pick_first delegation

[shivaspeaks]

Summary of Changes This pull request refactors the grpclb load balancer's PICK_FIRST mode to delegate its logic to a standard pick_first load balancing policy.

The key changes are as follows:

1. grpclb/build.gradle

Added dependency on grpc-util module to access ForwardingLoadBalancerHelper

2. grpclb/src/main/java/io/grpc/grpclb/GrpclbState.java

• New imports: LoadBalancer, LoadBalancerProvider, LoadBalancerRegistry, ResolvedAddresses, FixedResultPicker, ForwardingLoadBalancerHelper
• New fields for PICK_FIRST delegation:
  • pickFirstLbProvider - Provider for creating child pick_first LB
  • pickFirstLb - The child LoadBalancer instance
  • pickFirstLbState / pickFirstLbPicker - Track child LB's state and picker
  • currentPickFirstLoadRecorder - Load recorder for token attachment
• Key behavioral changes:
  • updateServerList() PICK_FIRST case: Instead of creating a single subchannel, it now:
    • Creates the child pick_first LB once and then updates it with new addresses on subsequent updates.
    • Passes addresses to child LB via acceptResolvedAddresses()
  • maybeUpdatePicker() PICK_FIRST case: Uses child LB's state and picker wrapped with ChildLbPickerEntry
  • RoundRobinEntry.picked() signature change: Changed from picked(Metadata) to picked(PickSubchannelArgs) to allow child picker delegation
  • New ChildLbPickerEntry class: Wraps child LB's picker and attaches TokenAttachingTracerFactory for token propagation
  • New PickFirstLbHelper class: Forwarding helper that intercepts updateBalancingState() to store child state and trigger grpclb picker updates
  • Updated shutdown(), requestConnection(), maybeUseFallbackBackends(): Handle the new child LB delegation model

3. grpclb/src/test/java/io/grpc/grpclb/GrpclbLoadBalancerTest.java

• Updated tests to reflect the new delegation behavior:
  • Initial state is now CONNECTING (not IDLE) since standard pick_first eagerly connects
  • Tests now verify the child LB is created only once and then handles address updates internally.
  • Adjusted verification expectations for the new flow
• Key Behavioral Changes:
  • Delegation to child LB: The grpclb state no longer directly manages subchannels for PICK_FIRST mode; the child pick_first LB handles this internally.