Add new API method for recordCustomEvent()

[elucus]

When I want to record a metric or an error, I just do NewRelic.recordMetric() or NewRelic.noticeError(). But if I want to report a custom event, I have to do NewRelic.getAgent().getInsights().recordCustomEvent(). The Insights product has been deprecated for years and events have been considered a key telemetry type for longer than that. It seems like it's probably time to make it just as easy to record events with the agent as it is to record metrics or errors.

When we add this we should mark the old method as obsolete.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-346977