[WIP] Add Qubit Mixer submodule

[JerryChen97]

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[JerryChen97]

Thoughts: qutrit_mixed already provided a very good example in generalization by replace hardcoded local hilbert space dim with QUDIT_DIM. Therefore here I would like to just copy paste qutrit_mixed to qubit_mixed to serve as a template for further usage. However, we would like to also investigate if there's any special design for qubit_mixed during the dev of new default mixed compatible with new API