Add emitConfigChangeEvents option back

adds back the config option emitConfigChangeEvents
it was removed in https://github.com/jenkinsci/datadog-plugin/pull/364 because it is no longer supported, but some users still have it configured. When they upgrade the plugin, their configuration will break.
fixes https://github.com/jenkinsci/datadog-plugin/issues/372
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Review checklist (to be filled by reviewers)
- [ ] Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
- [ ] PR title must be written as a CHANGELOG entry (see why)
- [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
- [ ] PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
- [ ] PR should not have do-not-merge/ label attached.
- [ ] If Applicable, issue must have kind/ and severity/ labels attached at least.

jenkinsci / datadog-plugin