Add a new jobtap plugin to select from multiple Flux clusters and delegate to the best one.

[tpatki]

Policies included in this version:

1. Random selection
2. Shortest Match Time
3. Least Pending Jobs

ToDo:

• Add documentation on the select_and_delegate plugin, along with build instructions, construction of clusters.json and a description of the policies and associated scripts.
• Clean up and format code
• Test with valgrind and address any memory leaks (can be done later as a separate issue after merging)