feat: support layer 4 load balancing

[jranson]

Description

This change expands the ALB to support Layer 4 load balancing for both TCP and UDP and adds several new mechanisms to the ALB.

Fixes #1145

New Mechanisms

Mechanism	Config	Provides	Description
Power of Two Choices	p2c	Scaling	draws two healthy pool members at random and routes to the one with fewer requests in flight
Least Connections	lc	Scaling	routes to the healthy pool member with the fewest requests in flight
Least Time	lt	Speed	routes to the healthy pool member with the lowest response latency, scaled by its requests in flight
Highest Random Weight	hrw	Affinity	consistently routes each client, tenant or other key to the same healthy pool member
Connect Race	race	Speed	connects a tcp or tls stream connection to several pool members at once and relays over the first to connect
UDP Mirror	udp_mirror	Replication	copies every datagram of a udp stream session to every healthy pool member, and answers from the first

Type of Change

• • Bug fix
• • New feature
• • Optimization
• • Test coverage
• • Documentation
• • Infrastructure

AI Disclosure

• • This contribution DOES NOT include AI-generated changes
• • This contribution DOES include AI-generated changes, and I have reviewed the relevant contributing guidelines.