feat: Implement streaming for Functionary v2 + Bug fixes (#1419)

* set up streaming for v2

* assert v2 streaming, fix tool_call vs function_call

* fix streaming with tool_choice/function_call

* make functions return 1 function call only when 'auto'

* fix

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Co-authored-by: Andrei <abetlen@gmail.com>

Author: jeffrey-fong