bug: old api-key version for New-Relic service

Now new-relic service uses old api key: https://github.com/argoproj/notifications-engine/blob/master/pkg/services/newrelic.go (line 153) `req.Header.Set("Api-Key", s.opts.ApiKey)`

According to documentation it should be `req.Header.Set("X-Api-Key", s.opts.ApiKey)` (https://docs.newrelic.com/docs/apis/rest-api-v2/get-started/introduction-new-relic-rest-api-v2/)

Now newrelic module doesn't create deployment markers. Same request with curl works (because of type of header).

Open pkg/services/newrelic.go around line 153 and compare the request header with the linked New Relic REST API documentation. Update the request so the New Relic module can create deployment markers, then verify that deployment-marker requests succeed.

Written by the indexing model from the issue text.