Add heartbeat-based dynamic profiling system

This PR introduces a heartbeat protocol that enables dynamic profiling
control. Agents periodically send heartbeats to a Performance Studio
backend and receive start/stop profiling commands, allowing on-demand
profiling without agent restarts.

Key features:
- HeartbeatClient for server communication
- DynamicGProfilerManager for profiler lifecycle management
- Command idempotency to prevent duplicate execution
- Support for dynamic profiler configuration
- PerfSpect hardware metrics integration
- Comprehensive test suite with mock and live modes
- Complete documentation with examples

Files added:
- gprofiler/heartbeat.py (627 lines)
- docs/HEARTBEAT_SYSTEM_README.md (634 lines)
- tests/test_heartbeat_system.py (358 lines)
- tests/run_heartbeat_agent.py (136 lines)

Files modified:
- gprofiler/main.py (heartbeat initialization)

Source: Pinterest's gprofiler repository
Testing: Mock tests pass, live tests verified with backend

Author: ashokbytebytego