[ENHANCEMENT] Define a generic diagonalarray constructor

[mtfishman]

Define a generic DiagonalArrays.diagonalarray constructor. For standard inputs, like diagonalarray(randn(2), (2, 2)), it will construct a DiagonalArrays.DiagonalArray, but for special kinds of axes like named axes it can construct a NamedDimsArrays.NamedDimsArray or ITensors.ITensor wrapping a DiagonalArray.

Analogous to DiagonalArrays.diagonal introduced in #4 as a generalization of LinearAlgebra.Diagonal.