feature: nested json object parsing

[Med951]

I added the following features:

Supporting nested json object parsing, changes are in message.go. When monitoring different Variables Grafana does not lose the old values, changes are in message.go and datasource.go. Update message_test.go to test the new added features.

Please review and let me know what you think!

[CLAassistant]

All committers have signed the CLA.

[bmedici]

Hello, this feature is really useful for complex structures. What's current status of the merge request or the feature itself ? Thank you !