Data factory missing encode support for timedelta

[yunhaoling]

The data type datetime.timedelta is not supported in uamqp when encoding message (or its properties) leading to the failure of sending messages containing timedelta.

In .Net AMQP implementation, they support TimeSpan by a DescribedType.
https://github.com/Azure/azure-sdk-for-net/blob/94b8c933793f0801aa97d80218798894fa32d74f/sdk/servicebus/Azure.Messaging.ServiceBus/src/Amqp/AmqpMessageConverter.cs#L554-L555

The descriptor is com.microsoft::timespan, and the value is integer (seconds).
We should investigate whether this could be supported in uamqp.

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we should support

msg = Message(body='data')
msg.application_properties = { 'some_key': timedelta(hours=1') }  # timedelta data type

SendClient.send_messages(msg)