ITelemetry.SerializeData instead of Serialize

[cijothomas]

Make ITelemetry implement SerializeData instead of Serialize, as only Data part need to be serialized. Rest of the content are known within ITelemetry itself and hence require no explicit serialization call.

So if user want to define a new type, implement ITelemetry. (which include SerializeData - to let channels know what fields are defined by the user. All channels should know how to serialzie the rest of the content as they are on ITelemetry itself.)

• I ran Unit Tests locally.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #Strong typed extensibility for Telemetry types #871
• Changes in public surface reviewed
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue.
• The PR will trigger build, unit test, and functional tests automatically. If your PR was submitted from fork - mention one of committers to initiate the build for you.

[cijothomas]

Will move duplicate code to a common method.