feat: support custom ctx key to pass to downstream in Service-Inline

[Duslia]

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feat

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在合并编译场景下，支持特定的 ctx key 传递给下游

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en: Service-Inline is a form of ByteDance's internal service integration, running the upstream and downstream within the same process.. However, to ensure isolation and consistency with RPC calls, the ctx key passed during Service-Inline must conform to the metainfo specification. However, since it's within a single process, data can be stored within a specific key, and the downstream can simply assert this structure for use. This reduces serialization compared to passing data via IDL.
Note: Due to package isolation, using this feature requires ensuring that the package of the passed structure is isolated.

zh(optional): 合并编译是字节跳动内部服务合并的一种形态，将上游和下游运行在同一个进程内。但是为了隔离性以及与 RPC 调用时的一致性，合并编译传递的 ctx key 需要满足 metainfo 规范。然而既然在一个进程内，可以将一些数据塞到特定的 key 中，下游直接断言出来这个结构体使用就可以。相比通过 IDL 传递来说，可以减少序列化的次数。
注意：由于包的隔离性，使用该 feature 时必须保证传递的结构体的包时可以被隔离的。

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.37%. Comparing base (3c0b00f) to head (b01196e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1709   +/-   ##
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  Coverage   61.36%   61.37%           
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  Files         388      388           
  Lines       35028    35030    +2     
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+ Hits        21496    21498    +2     
+ Misses      12243    12239    -4     
- Partials     1289     1293    +4     

Flag	Coverage Δ
integration	50.40% <0.00%> (+0.13%)	⬆️
unit	51.50% <100.00%> (-0.07%)	⬇️

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