Rename functions/classes for elementwise unary operations

Rename functions/classes for elementwise unary operations

* Create CpuElementwiseUnary operator
* Rename kernel classes
* Make the kernels stateless

Partially implements: COMPMID-4003

Change-Id: Ie0440cd01d4924847d6991b4df7ccaf311439297
Signed-off-by: Sang-Hoon Park <sang-hoon.park@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4912
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Reviewed-by: Georgios Pinitas <georgios.pinitas@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>