Add custom events support into integration package

[morningspace]

The integration package currently does not support custom events, which is one of the important features of Instana. We should also add custom events into the package, that allows customer to define custom events for the technology being integrated or monitored.

The CLI will also need to be modified to support this custom element. Essentially, it will need to talk to Instana backend via REST API to post user defined custom events into Instana system.