Extract ORCA logic from CSWRR into reusable library

[jukie]

This is related to #43665 and is part 1 to enable adding new ORCA-based LB policies without duplicating CSWRR internals.

Extracts ORCA weight management logic from ClientSideWeightedRoundRobinLoadBalancer into a reusable OrcaWeightManager class. This is a refactor with no behavior change and all existing CSWRR tests pass.

• OrcaWeightManagerConfig — config struct for timing params, metric names, and error penalty
• OrcaLoadReportHandler — computes host weight from ORCA load reports
• OrcaHostLbPolicyData — per-host LB data (weight + timestamps), renamed from ClientSideHostLbPolicyData
• OrcaWeightManager — orchestrates periodic weight updates via timer and priority-set callbacks

AI disclosure: AI agents were used during implementation and for writing tests but I fully understand the changes here

Commit Message: Extract OrcaWeightManager from ClientSideWeightedRoundRobinLoadBalancer into reusable common library
Additional Description: Extracts ORCA internals from CSWRR to allow for more ORCA-based LB policies without duplication
Risk Level: Low, clean extraction without any behavior changes
Testing: New test suite added, all existing CSWRR tests pass unchanged, and WrrLocality tests pass
Docs Changes: No user-facing changes or docs update needed
Release Notes: None
Platform Specific Features: N/A
xRef: #43665

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Caused by: #43695 was opened by jukie.

see: more, trace.