Default to user agent for normalized_os if no os field is present

[jklukas]

In ParsePayload, we look for an OS name inside the ping using known locations for common ping, glean, and core ping. Any other payload type gets a null normalized_os value.

It would be nice to support normalized_os out of the box for structured ingestion by falling back to the OS name parsed out of the user agent string.

This may require some additional logic in NormalizeAttributes if the values we see in UA strings differ from the values reported in telemetry payloads.