Update utilization API to build on cpu-only systems, rename API to be generic

[tpatki]

Description

Fixes #518

To Do:

• CPU-only build utilization numbers
• Check for platform and call the GPU function if available
• Rename the API from variorum_get_node_utilization_json to variorum_get_utilization_json
• Remove the variorum_get_gpu_utilization_json API
• Check if gpu_utilization is also removed from docs

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature/architecture support (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Build/CI update

How Has This Been Tested?

• Lassen-gpu-only
• Lassen-cpu-only
• Lassen-both
• Corona-gpu-only
• folsom
• thompson
• comus
• rhetoric
• Juno r2

Checklist:

• I have run ./scripts/check-code-format.sh and confirm my code code follows the style guidelines of variorum
• I have added comments in my code
• My changes generate no new warnings (build with -DENABLE_WARNINGS=ON)
• New and existing unit tests pass with my changes