CTest Resource Allocation: Add file version, clarify some things in the documentation
This MR does the following:
- Add a versioning scheme to the arguments to get a resource spec file
- Add a versioning scheme to the resource file itself
- Clarify that tests that require resources can still run even without a resource spec file
- Clarify that tests that can tolerate slots from multiple instances of the same resource can split it up accordingly
Fixes: #19985 (closed)
Fixes: #19987 (closed)
Fixes: #20007 (closed)
Backport: release