Multiple workloads with same policy

This issue is related to #23614.

Right now users can set the job, group and task on the policy when they want an allocation to get custom permissions on the nomad API. This is OK for most cases, but when you need several jobs to have the same permissions (same policy), we're forced to create another policy with the same rules but another name to be able to set the job, group...

I think there should be a separate table with a mapping of policy -> workload for nomad policies (or at least have a list of mappings in the policy). Maybe there could be an API endpoint to add WI to ACL policies.

Start by reading related issue #23614 and the current Nomad ACL policy and workload mapping behavior. Define the API or policy mapping change needed to let multiple workloads share one policy, with completion demonstrated by several workloads receiving the same permissions without duplicate policies.

Written by the indexing model from the issue text.