[Core] In stream upload, RetryPolicy should rewind the body position if possible, or throw

[lmazuel]

That's actually done in storage already, and should be part of the main Retry policy:
https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/storage/azure-storage-blob/azure/storage/blob/_shared/policies.py#L496

[lmazuel]

Doing this like TDD, we need the following tests:

• We create a request and use "set_streamed_data_body" or we use "stream_content" kwarg (both are equivalent), we hack the transport to read part of this body, and we raise an exception, and we check when we come back we can read the whole body.
• We create a request and use "set_form_data" or we use "form_content" (both are equivalent), with content-type header 'multipart/form-data' (file upload). We hack the transport to read part of the file descriptor, and we raise an exception, and we check when we come back we can read the whole file. Create a tmp file with "tmpfile"

set_streamed_data_body:

azure-sdk-for-python/sdk/core/azure-core/azure/core/pipeline/transport/_base.py

Lines 285 to 290 in 3e878ff

	def set_streamed_data_body(self, data):
	"""Set a streamable data body.
	:param data: The request field data.
	:type data: stream or generator or asyncgenerator
	"""

stream_content kwarg:

azure-sdk-for-python/sdk/core/azure-core/azure/core/pipeline/transport/_base.py

Line 720 in 3e878ff

stream_content=None, # type: Any

set_form_data:

azure-sdk-for-python/sdk/core/azure-core/azure/core/pipeline/transport/_base.py

Lines 326 to 331 in 3e878ff

	def set_formdata_body(self, data=None):
	"""Set form-encoded data as the body of the request.
	:param data: The request field data.
	:type data: dict
	"""

form_content kwarg:

azure-sdk-for-python/sdk/core/azure-core/azure/core/pipeline/transport/_base.py

Line 719 in 3e878ff

form_content=None, # type: Optional[Dict[str, Any]]

Example of usage of both can be found here:
https://github.com/Azure/autorest.python/blob/azure-core/test/vanilla/Expected/AcceptanceTests/BodyFormData/bodyformdata/operations/_formdata_operations.py

In term of implementation, the storage one is fine for the first problem, but doesn't cover the second one. It's ok for storage, since they don't do multipart/form-data

[xiangyan99]

fixed