[ate]: Enable client-side load balancing and update docs

[moidx]

This change introduces gRPC client-side load balancing to the ATE client library and documents its usage.

ATE Client Library:

• The client options now accept a pa_target in gRPC name-syntax format (e.g., "ipv4:host1:port,host2:port") to enable connecting to multiple server instances.
• A load_balancing_policy option has been added to allow callers to select policies like "round_robin".
• The client creation logic was updated to use grpc::CreateCustomChannel to apply the specified load balancing configuration.
• Test programs and integration test scripts were updated to use the new --pa_target and --load_balancing_policy flags.

Documentation (docs/ate.md):

• Added a section on "Client-Side Load Balancing and Failover," explaining how to configure it and what to expect during partial and total server outages.
• Added a "Client Lifecycle and Resource Management" section outlining best practices for creating and destroying the client instance.
• Added a "Monitoring and Debugging" section that instructs users on how to enable gRPC tracing via environment variables to debug connection health and load balancer behavior.