Add hcat, vcat features

[MaxenceGollier]

@dpo it would be great if we could have something like in LinearOperators.jl for concatenating matrices.
This would then be useful for the Golub-Riley implementation in QRMumps.jl where we want to work on a matrix defined as

$$A_{\alpha} = \begin{bmatrix}A & \alpha I \end{bmatrix}$$

[MaxenceGollier]

I somehow didn't see it was already implemented in coo_linalg.jl, my bad.