Replace MulAddMul by alpha,beta in __muldiag

[jishnub]

This PR replaces MulAddMul arguments by alpha, beta pairs in the multiplication methods involving Diagonal matrices, and constructs the objects exactly where they are required. Such an approach improves latency.

julia> D = Diagonal(1:2000); A = rand(size(D)...); C = similar(A);

julia> @time mul!(C, A, D, 1, 2); # first-run latency is reduced
  0.129741 seconds (180.18 k allocations: 9.607 MiB, 88.87% compilation time) # nightly v"1.12.0-DEV.1505"
  0.083005 seconds (146.68 k allocations: 7.442 MiB, 82.94% compilation time) # this PR

julia> @btime mul!($C, $A, $D, 1, 2); # runtime performance is unaffected 
  4.983 ms (0 allocations: 0 bytes) # nightly
  4.938 ms (0 allocations: 0 bytes) # this PR

This PR sets the stage for a similar change for Bidiagonal/Tridiaognal matrices, which would lead to a bigger reduction in latencies.