Add annotator workload balancer example

[drinman]

Annotator Workload Balancer Example

This PR adds a new example script that demonstrates how to use the Label Studio SDK to automatically detect and balance annotator workloads.

Features

• Monitors distribution of pending tasks among annotators
• Automatically reassigns tasks when workload imbalance exceeds threshold
• Configurable check intervals and rebalance thresholds
• Detailed logging of workload status

File Structure

• Added examples/annotator_workload_balancer/ directory
• Added Python script and README with usage instructions

This example follows the existing folder structure pattern and provides a practical utility for Label Studio users managing large annotation projects with multiple annotators.