Create variorum_json.h/c

[tpatki]

Create a separate header/c for JSON APIs to allow for code clarity.

Several use cases with Caliper and Flux integrations are coming up, requiring us to extend the current JSON support to include other APIs. We may have to break this issue down into further sub-issues, but capturing the key idea here.

• Add variorum_get_energy_json
• Add variorum get_node_util_json
• Add variorum_get_thermals_json
• Add variorum_get_gpu_util_json