Pagerduty plugin cannot handle an escalation policy entry that contains a user instead of a schedule

[ghost]

Describe the bug
If a pagerduty escalation policy entry does not contain a schedule, Dispatch fails to find the appropriate contact.

To Reproduce
Steps to reproduce the behavior:
Create a Pagerduty escalation policy similar to the following

Attempt to report an incident. The incident will not be created. Check the logs and you will see:

ERROR:No users could be found for this pagerduty escalation policy (*snip1*). Is there a schedule associated?:/usr/local/lib/python3.8/site-packages/dispatch/incident/views.py:create_incident:112         
Traceback (most recent call last):                                                                    
  File "/usr/local/lib/python3.8/site-packages/dispatch/incident/views.py", line 110, in create_incident                                                                                                    
    incident = create(db_session=db_session, **incident_in.dict())
  File "/usr/local/lib/python3.8/site-packages/dispatch/incident/service.py", line 228, in create
    assign_incident_role(
  File "/usr/local/lib/python3.8/site-packages/dispatch/incident/service.py", line 44, in assign_incident_role
    assignee_email = oncall_plugin.instance.get(service_id=service.external_id)
  File "/usr/local/lib/python3.8/site-packages/dispatch/decorators.py", line 106, in wrapper
    result = func(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/dispatch/decorators.py", line 122, in wrapper
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/dispatch/plugins/dispatch_pagerduty/plugin.py", line 33, in get
    return get_oncall(client=client, service_id=service_id)
  File "/usr/local/lib/python3.8/site-packages/dispatch/plugins/dispatch_pagerduty/service.py", line 35, in get_oncall
    return get_oncall_email(client, service)
  File "/usr/local/lib/python3.8/site-packages/dispatch/plugins/dispatch_pagerduty/service.py", line 24, in get_oncall_email
    raise Exception(
Exception: No users could be found for this pagerduty escalation policy (*snip1*). Is there a schedule associated?
INFO:     10.84.80.225:39996 - "POST /api/v1/default/incidents HTTP/1.1" 400 Bad Request

Expected behavior
get_oncall_email() should be aware of the possibility of an escalation policy entry that contains a user instead of a schedule. The exception message implies that a schedule is the only valid input in this spot, but that's not true.

Additional context
Running dispatch version from PR #1493
@ymatsiuk is under the red box in the screenshot

[ymatsiuk]

In current plugin implementation we only filter on schedules, expecting that the first element of the escalation policy is schedule.

This is what PD's /escalation_policies API endpoint returns:

    "escalation_rules": [
      {
        ...
        "targets": [
          {
            "type": "user_reference",
            "summary": "Yurii Matsiuk",
            ...          }
        ]
      },
      {
        ...
        "targets": [
          {
            "type": "schedule_reference",
            "summary": "SRE Schedule Primary",
            ...
          }
        ]

As a quick workaround we can introduce conditional here to check for escalation_policy["escalation_rules"][0]["targets"][0]["type"] and if user_reference than replace this with user_ids[]