Multiple Policies to Govern a single Action/Workflow

I've noticed that it's not possible to have multiple policies for a single action/workflow. This is something that would be really useful. Imagine a workflow that has a user and host input, token example. I'd like to be able to do something like:

policies/my_action_hosts.yaml

name: my_pack.my_action.host.concurrency
description: control concurrency of `my_action` by host
resource_ref: my_pack.my_action
parameters:
  action: cancel
  threshold: 1
  attributes:
    - hosts

policies/my_action_user.yaml

name: my_pack.my_action.host.concurrency
description: control concurrency of `my_action` by host
resource_ref: my_pack.my_action
parameters:
  action: cancel
  threshold: 5
  attributes:
    - user

@cognifloyd @nzlosh do y'all think this is something that might be useful? I'd be interested in contributing to it, if it's possible. Would need to get a dev environment for st2 proper set up.

The issue names policy YAML files for a single action/workflow and separate host and user attributes, but no source files or tests. Set up a st2 development environment, find the policy-loading and enforcement entry points, and trace how one resource reference is handled. Done means multiple policies can govern one action/workflow with their own thresholds and attributes, with tests covering the examples.