Migrate PagerDuty plugin from Graylog Labs repo to official open source integrations repo

[waab76]

As a Graylog engineer, I want to migrate the PagerDuty plugin from the GraylogLabs repo to the official Graylog plugins repo so we can begin officially supporting it.

Input Criteria:

• Working version of PagerDuty plugin in GraylogLabs repo

Tasks:

• Migrate PagerDuty plugin to Graylog plugins repo
• Ensure PagerDuty plugin follows Graylog coding standards
• Ensure PagerDuty plugin has appropriate unit tests
• Create official documentation for PagerDuty plugin

Acceptance Criteria:

• PagerDuty plugin included in next Graylog release

[waab76]

We should try to ensure in the migration that we take care of this issue: graylog-labs/graylog-plugin-pagerduty#18

It looks like PagerDuty is using apache for HTTP connections instead of OKHttp, so this may get fixed in the switch to OKHttp

[waab76]

Merged to 4.0