Compute how large a D2C message will be

[drwill-ms]

Is your feature request related to a problem? Please describe.
Give the user feedback about how large a message will be so it can be taken into account for message batching.

Describe the solution you'd like
It needs to take into account all the payload that the service uses to determine message size, some of which is not available to the user.

Additional context
User (@Marusyk) asking for this feature here: Azure/azure-sdk-for-net#16969.

[barnstee]

Using the full payload available for a large IoT Hub message right now is a guessing game as the system property size is not exposed but subtracted from the maximum available 256K message size. Please implement this feature request.