feat: add configurable quantized indexer cache support.

[phantomlei3]

Description

Adds configurable INT8 quantization for indexer caches on the MLU backend via --indexer_cache_dtype. The change covers cache allocation and capacity estimation, scale tensors, distributed/RDMA transfer, worker configuration propagation, documentation, and comprehensive tests.

为 MLU 后端新增可配置的 Indexer Cache INT8 量化能力，通过 --indexer_cache_dtype 参数启用。相关修改涵盖缓存分配与容量估算、量化缩放因子、分布式/RDMA 传输、Worker 配置传递、文档及完整测试。GLM5.1 indexer int8在pd分离下支持，decode阶段在32k上下文以上有收益，prefill阶段在4k上下文以上存在收益。

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactor
• Documentation
• Test
• Build or CI

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