math/big: big.Int JSON marshalling to/from string

[rubensayshi]

big.Int by design marshalls / unmarshalls to a number in JSON but for actual BIG numbers this ends up with a mess in other languages, specially javascript ...

Now encoding/json has a ,string struct tag to flag a field to be quoted as a string instead but it's restricted only for strings, floats, integers, and booleans can be quoted. atm.
If this could also be applied to big.Int then we'd have a good solution for BIG numbers without breaking the existing functionality at all!

related; #28154
and there's probably many more people running into this issue...

[AlexanderYastrebov]

Somewhat related to #34472

[odeke-em]

Thanks for the patience everyone over the years and thanks for the feature request! So this feature request came up before the proposal requirements were strong, thus perhaps let's convert this into a quick and simple proposal and then we can have a go ahead for it @rubensayshi!