APM: initial FilterManager implementation without offloaded field streaming support

[penguingao]

Commit Message:
Initial implementation of APM's FilterManager that coordinates sending the parsed JSON object through AI filters. This version only supports streaming the JSON object itself, without the ability to stream the offloaded payload through the filter chain yet. The offloaded payload streaming is a TODO.

We also add re-serialization, so that a JSON object can be normalized / transcoded by the filter chain.

There is desire to run two instances of APM in legacy deployments, where one APM runs at early point of downstream filter chain, one runs on the upstream filter chain. The idea is to be able to perform other normal HTTP functionalities after APM, and once the routing decision is made, an upstream filter performs transcoding to the final model provider. To support this, and help filters between the two APMs to access the parsed JSON, we provide a JsonWithExtBuf that correctly indexes the payload that goes through the filters. This index is also useful for the later APM to avoid another pass of parsing.

This is part of #44681

Additional Description: https://github.com/penguingao/thoughts/blob/main/20260727_AI_FILTER.md
Risk Level: medium
Testing: unit tests
Docs Changes: n/a WIP filter
Release Notes: n/a WIP filter
Platform Specific Features: no

[repokitteh-read-only]

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🐱

Caused by: #47119 was opened by penguingao.

see: more, trace.

[penguingao]

/assign @botengyao @tyxia @wbpcode