Fix backward compatibility of Conversation
Author: wdhorton
Pull request: yes



# What does this PR do?
I ran into a case where an external library was depending on the `new_user_input` field of Conversation. https://github.com/SeldonIO/MLServer/blob/release/1.4.x/runtimes/huggingface/mlserver_huggingface/codecs/utils.py#L37 

This field was deprecated as part of the refactor, but if `transformers` wants to maintain backwards compatibility for now (which is mentioned in a few comments) then there's a good argument for supporting it. Some comments referred to it as an "internal" property, but it didn't start with `_` as is Python convention, so I think it's reasonable that other libraries were referencing it directly.

It's not difficult to add it to the other supported backwards-compatible properties. In addition, the implementation of `past_user_inputs` didn't actually match the past behavior (it would contain the most recent message as well) so I updated that as well.
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Comments:
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