[servicebus] add repr() to ServiceBusMessage/ServiceBusReceivedMessage/ServiceBusBatchMessage

[rakshith91]

latest updates:

• we decided to pause the support for body_as_str method until there're customer requests.
• The ServiceBusMessage (and other message model) should implement the repr method as per our guideline here: https://azure.github.io/azure-sdk/python_design.html#python-models-repr

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right now, msg.body returns a generator since it's directly returning the data object from the uamqp message object.

It means to extract contents as a string I have to do next(msg..body).decode('utf-8')

body_as_Str would be a nice convenience to add ; there's on in EH too

[yunhaoling]

hey Rakshith Bhyravabhotla (@rakshith91) , I had a brief discussion with anna on this topic, some feedback:

• in python str(message) could be used and is the preferred way in python to get a string of the object.
  • call str() on a ServiceBusMessage will get the str.
• the value that the method body_as_str could provide is taking the encoding parameter to decode the bytes.

Question for you:
do you know if there's any use case for SB or EG that has the need to decode a ServiceBusMessage with an encoding different than utf-8?
If the built-in str() method could satisfy the need, then we could push this request until there're customer requests.