FocusColor option in OtpFieldStyle

[sina-moradbakhti]

Dear Owner and other contributors

First I appreciate you for creating such a useful and clean dependency.

As the dependency is really popular, lots of people are using that, based on my experience in many project we need to have another option for changing the focus color regarding the OTP field.

based on my investigation on your code, there is OtpFieldStyle which we can use for put another field with name, focusColor beside the fillColor, which help us to have focus Color as well, then use that inside the textInputDecoration in the mainClass.

I forked the repo and do it, I'd be thankful if you let me know if its utilized and you can merge it in maste r branch or not?

Regards You,
Sina