[RUST] Implement value function with Array trait

[Fernando Herrera]

Hi,

I'm wondering if it has been considered to move the value function that is
implemented in all the arrays (StringArray, BooleanArray, ListArray, etc)
as part of the Array trait?

This would help when extracting values from generic arrays that implement
dyn Array without having to manually downcast the array all the time to
read a value from the array.

Thanks,