Implement connection health monitoring and intelligent load balancing

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### Description

## Summary
Implement comprehensive connection health monitoring and intelligent load balancing for py-libp2p to address the 1-1 connection mapping limitation.

## Related Discussion
- Discussion #717: TODO Analysis - Swarm.py Issues

## Scope
- New `libp2p/network/health/` module
- Integration with Swarm connection lifecycle
- Extended `ConnectionConfig` with health monitoring options
- Host-level health monitoring API methods

### Motivation

## Goals
- Track connection health metrics (latency, success rates, bandwidth, stability)
- Proactive health monitoring service with configurable intervals
- Multiple load balancing strategies (round_robin, least_loaded, health_based, latency_based)
- Health-aware connection selection
- API consistency fix for `new_host()` to accept `connection_config` parameter

### Current Implementation

## Related Discussion
- Discussion #717: TODO Analysis - Swarm.py Issues

### Are you planning to do it yourself in a pull request ?

Yes

Start with Discussion #717 and inspect the existing Swarm connection lifecycle, ConnectionConfig, and new_host() entry points. Review the proposed libp2p/network/health/ module boundary and identify existing tests for connection management before defining the monitoring and load-balancing behavior. Done should include the health API, lifecycle integration, configurable strategies, and coverage for the documented goals.

Written by the indexing model from the issue text.