Include millisecond component (%f) in datetime serialization

https://github.com/Azure/azure-storage-python/blob/4306898850dd21617644fc537a57d025e833db74/azure-storage-common/azure/storage/common/_serialization.py#L46

Milliseconds are being stripped from datetime values when serializing:

return value.strftime('%Y-%m-%dT%H:%M:%SZ')
-->
return value.strftime('%Y-%m-%dT%H:%M:%S.%fZ')

Open azure-storage-common/azure/storage/common/_serialization.py at line 46 and inspect the datetime serialization path. Update the format so fractional-second data is retained, then verify that serialized datetime values include the expected millisecond component.

Written by the indexing model from the issue text.