generic `mutual_information` function

[Fe-r-oz]

There is a need for a generic function mutual_information in QuantumInterface.jl to standardize its usage and ensure compatibility across related packages. This function is referenced in QuantumSavory/QuantumClifford.jl#338. As advised by @Krastanov, the goal is to 'pick a name that can be reused by other packages'.

[akirakyle]

Might be a good excuse to start a metrics.jl file that defines and documents such generic functions representing "metrics on quantum objects" (for example entanglement_entropy would move there). I think trying to be explicit about any conventions that downstream packages should observe so that there's some notion of compatibility between different implementations is the most important information to put in the doc strings here. One obvious interface task here is specifying the units (e.g. bits versus nats).