add output format option to `adjacency_matrix` arguments

[CarloLucibello]

We should add a keyword argument adjacency_matrix(g, fmt=...) allowing to choose among dense and sparse matrix formats:

• :coo:
  • on cpu return a sparse matrix from SparseMatricesCOO.jl
    if the package is loaded (otherwise error out)
  • on cuda return a CUSPARSE. CuSparseMatrixCOO
  • on metal and amdgpu: ?
• :csc
  • on cpu return SparseArrays. SparseMatrixCSC
  • on cuda return CUSPARSE. CuSparseMatrixCSC
• :dense: return a dense array
• :nothing: maintain the current behavior (which ideally should be to choose the most sensible output format based on the graph format).

Other options (e.g. :csr could be considered in the future).

cc @dferre97