Add pagerduty_business_service_subscriber resource

[gsreynolds]

TF_ACC=1 go test -run "TestAccPagerDutyBusinessServiceSubscriber" ./pagerduty -v -timeout 120m     
=== RUN   TestAccPagerDutyBusinessServiceSubscriber_import
--- PASS: TestAccPagerDutyBusinessServiceSubscriber_import (32.39s)
=== RUN   TestAccPagerDutyBusinessServiceSubscriber_User
--- PASS: TestAccPagerDutyBusinessServiceSubscriber_User (25.49s)
=== RUN   TestAccPagerDutyBusinessServiceSubscriber_Team
--- PASS: TestAccPagerDutyBusinessServiceSubscriber_Team (25.30s)
=== RUN   TestAccPagerDutyBusinessServiceSubscriber_TeamUser
--- PASS: TestAccPagerDutyBusinessServiceSubscriber_TeamUser (27.49s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   111.033s

Closes #390

[stmcallister]

Very nice! Thank you!! 🎉 🎉

[stmcallister]

@gsreynolds before we merge this, can you update the reference for the go library?

go get -u github.com/heimweh/go-pagerduty

go mod vendor

And then push those changes here?

[gsreynolds]

Hey @stmcallister, thanks. Rebased against master to resolve merge conflicts. Should be good to go!

[gsreynolds]

@stmcallister As discussed, I've submitted heimweh/go-pagerduty#73 to additionally check the response body for a successful subscription, as the API will return 200 OK if the result of one (or more) subscriptions in the request was not success. The endpoint is designed to accept multiple subscriptions for users and teams in one request, but here it is only being used to send one subscription in a single request.

I've also rebased the branch to catch up with latest changes, but no changes required in this PR other than updating the vendored packages once heimweh/go-pagerduty#73 is merged.

Example of 200 OK response body:

{
 "subscriptions": [
  {
   "account_id": "PQL6MPB",
   "result": "unauthorized",
   "subscribable_id": "P3D3U4T",
   "subscribable_type": "business_service",
   "subscriber_id": "PGW87TJ",
   "subscriber_type": "team"
  }
 ]
}

Fixes occasional errors resulting from non-success responses. If a non-success response is returned, the resource will retry as normal until retries are exceeded or it is successful.

    resource_pagerduty_business_service_subscriber_test.go:39: Step 1/1 error: Error running apply: exit status 1
        
        Error: Provider produced inconsistent result after apply
        
        When applying changes to pagerduty_business_service_subscriber.foo, provider
        "provider[\"registry.terraform.io/hashicorp/pagerduty\"]" produced an
        unexpected new value: Root resource was present, but now absent.
        
        This is a bug in the provider, which should be reported in the provider's own
        issue tracker.