Feature: Add sentry_organization_pagerduty_service resource

[sbrudz]

This PR adds a sentry_organization_pagerduty_service resource. It handles a single service/integration key at a time, hiding the fact that the API requires POSTing a full list of all the services/integration keys. Any service/integration keys that aren't managed by terraform will be ignored and kept as-is.

The resource has the form:

resource "sentry_organization_integration_pagerduty" "this" {
  organization    = "my-organization"
  integration_id  = "org-integration-id"
  service_name    = "name-of-pagerduty-service"
  integration_key = "integration-key-from-pagerduty"
}

I've added integration tests that pass on my test instances of Sentry + PagerDuty. Since this requires paid plans for both services and I wasn't sure if the terraform-provider-sentry's test environment has this, I made the tests require the SENTRY_TEST_PAGERDUTY_INTEGRATION_ID environment variable to be set in order to run. Otherwise, the tests will be skipped.

This PR depends on changes to the underlying go-sentry library to be merged and released (jianyuan/go-sentry#80). Once that's merged, I'll update this PR and switch the status out of draft.

Fixes #250

[vodila-privet]

I think it will be cool!!!

[alexandrKuzmitsky]

Any further actions, guys?

[jmesquita]

We really want this to land soon, how can we get it to move forward?

[sbrudz-orum]

I've resolved the merge conflict. It should be ready to go.

[rajesh-dhakad]

👍 We are waiting for this feature to be available.

[asibs]

Also waiting for this feature. Without it you need to:

1. Create a new PagerDuty Service and the PagerDuty side of the Sentry -> PagerDuty integration for the new PagerDuty Service (either in the PagerDuty UI, or using the PagerDuty Terraform module)
2. Manually go into the Sentry UI and sync any new PagerDuty Services to the PagerDuty integration
3. Use this Sentry Terraform provider to setup any alerts and push them to the relevant PagerDuty Service (either hardcoding the service integration id, or using some other mechanism to look it up such as making CURLs to the Sentry REST API)

The full process could be automated smoothly with this additional functionality in the Sentry module, so would be great to get it merged!