Support UniformScaling in multivariate normal constructors

[bartvanerp]

I was just running some code from a couple months ago and it seems that we lost the following functionality:

MvNormalMeanCovariance(randn(4), 100I)

It throws

MethodError: no method matching ExponentialFamily.MvNormalMeanCovariance(::Vector{Float64}, ::LinearAlgebra.UniformScaling{Int64})

In terms of user-friendliness the above syntax should be supported again, whilst maintaining the computational benefits of the UniformScaling struct.

[bvdmitri]

I've checked [email protected] before integration with ExponentialFamily.jl and it did work there as well, but it materialised the matrix (using the Diagonal type). So its definitely a regression. Can you work on it @bartvanerp ? If yes, I add it to the milestone.