feat(gpu): speed up packing KS for levels==1

[andrei-stoian-zama]

Optimize packing ks for level count == 1

• Adds an optimized GEMM kernel
• Adds an alternative packing keyswitch implementation using the gemm kernel
• Implements a fast path for packing keyswitch for levels==1 using the gemm kernel
• Adds the fast path to the integer compression GPU code and adds a test with custom parameters that use the fast path

Achieves ~4x speedup for packing keyswitch

[agnesLeroy]

Hei @andrei-stoian-zama! It looks good to me overall, but I think it would be good that @pdroalves take a look since he was the one to implement the packing keyswitch on GPU.