refactor: introduce inference tools

[louis-jan]

Describe Your Changes

• Extensions/assistants can now register the Inference Tool to be used across extensions/apps, which was not available previously. This eliminates the need for a hacky retrieval tool implementation (modifying & proxying requests), which added a lot of messy logic and code.

graph TD;
    AssistantExtension --> |registers Tools| CoreSDK;
    App --> |imports| CoreSDK;
    Extension --> |imports| CoreSDK;
    CoreSDK --> |uses| Tools;
    Tools --> |inference|Engines

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Fixes Issues

• epic: Refactor Inference Engines #2451

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed