[RFC] Add fundamental traffic governance building blocks in Sentinel 2.0 | 流量治理功能骨架与抽象

## Issue Description

Type: *feature request*

### Describe what happened (or what feature you want)

Sentinel 2.0 will evolve to a universal, cloud-native traffic governance component. Thus we'll need to add fundamental traffic governance building blocks in Sentinel 2.0, which at-least cover:

* A local "brain" for hinting load balancing #2944
* Traffic routing #2846
* Traffic tagging as well as carrying tags between services
* Outlier eviction

This is the umbrella issue of traffic governance building blocks in Sentinel 2.0.

Related issues: #2845 (regarding API model)

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Sentinel 2.0 将演进为通用、云原生的流量治理组件，并天然支持 OpenSergo 流量治理标准。Sentinel 1.x 的 API 与场景均基于流量策略来进行设计，而未干预流量到来之前的过程，以及中间的执行过程。在 Sentinel 2.0 中，我们需要扩充这一部分的抽象，来支持更多的流量治理能力，其中包括但不限于：

* 本地的决策组件，基于给定的流量与条件来为 load balancing 与 routing 提供 hint
* 流量路由流程的抽象
* 流量染色与标透传的流程抽象
* 异常实例摘除、熔断

Start by reading this umbrella issue and the related issues #2944, #2846, and #2845 to understand the proposed traffic-governance building blocks and API model. Before implementation, identify which capability has a defined scope; completion is not specified because this issue groups several Sentinel 2.0 design areas rather than naming one testable change.

Written by the indexing model from the issue text.