Allow hyper-parameter tuning for immutable models.

Some context: https://github.com/JuliaML/TableTransforms.jl/issues/67

I don't think this would be too bad, and useful preparation for making the MLJ model interface more flexible later.

The MLJTuning API doesn't really touch on this point. A tuning strategy needs to implement a models method to generate models to evaluate, but doesn't say how the models are generated. They needn't be mutations of a single object. However, the MLJ model interface currently states that models must be mutable, so some tuning strategies do use mutation to generate their models.

TODO:

• To see if the change would be breaking, update this table:

cc @juliohm

Start by reviewing the linked TableTransforms.jl issue and the MLJTuning API discussion, then inspect the listed tuning strategies and their providers. Determine which strategies assume mutable model types and update the compatibility table; done means the breaking-change status and required scope are documented for all listed strategies.