Add Exclusive Prefix Sum (Scan) Example

[SavannaSpayd]

There is no example that demonstrates an exclusive prefix sum (scan) using multiple execution policies. Such an example could implement the following features:

• Parallel Implementations: Versions that utilize RAJA's parallel execution policies (OpenMP and CUDA) to leverage multi-threading and GPU acceleration.
• Testing: Tests to compare performance against a serial baseline across implementations.
• Documentation: Doxygen comments integrated into the code to clearly explain functionality and usage.

[SavannaSpayd]

I would be happy to contribute such an example!

[SavannaSpayd]

@rhornung67 Just checking in to see your thoughts on this

[SavannaSpayd]

See PR #1825