Fix execution_resourceglobal vs. slot usage inconsistencies

[brynnz22]

I noticed in the documentation that execution_resource was showing up as having a range of string despite us implementing an ExecutionResourceEnum as its range in Berkeley. This was because the global definition of execution_resource had a range of string while under the WorkflowExecution class it had a range of ExecutionResourceEnum. I fixed this by:

• changing the range on the global definition of execution_resource from string to ExecutionResourceEnum
• I had to move the definition of ExecutionResourceEnum over to the same file execution_resource is globally defined on so it would pass the tests
• I kept the slot usage for execution_resource on WorkflowExecution to make it required.