[WIP][do not merge][CB] Support for heterogeneous tkv

[yannicks1]

[WIP][do not merge][CB] Support for heterogeneous tkv

First draft of heterogeneous CB, please do not review yet, this is very much work in progress:)

[github-actions]

👋 Hi! Thank you for contributing to vLLM support on Spyre.
Just a reminder: Make sure that your code passes all the linting checks, otherwise your PR won't be able to be merged. To do so, first install the linting requirements, then run format.sh and commit the changes. This can be done with uv directly:

uv sync --frozen --group lint --active --inexact

Or this can be done with pip:

uv pip compile --group lint > requirements-lint.txt
pip install -r requirements-lint.txt
bash format.sh

Now you are good to go 🚀