Add baudrate selection

[James4Ever0]

This would support third party KVM devices like MS2109 + CH9329/CH340, since it has a different baudrate (9600) than official (57600).

[wj-xiao]

Thank you so much for your hard work on this PR! The code is well-written, and we truly appreciate your contribution.

And, my apologies for the long delay in processing pull request.
We noticed that there is another open PR, #60 , which also addresses this feature. To keep the conversation organized, could we move the discussion over to that one?

Thanks again for your fantastic work!