Implement SensitiveString type

[qfrank]

Though I there's a drawback here. Adding a field that should be logged, will require user to keep in mind to add it to Marshal function.

I googled this solution, it looks much cleaner and scalable:

type SensitiveString string {}

func (SensitiveString s) String () string {
    return "****"
}

func (SensitiveString s) Value() string {
    return s
}

I guess we can merge as is and open an issue to fix all places for to use such approach.
cc @osmaczko

Originally posted by @igor-sirotin in #5677 (review)

[friofry]

as discussed on status-go guild: (https://docs.google.com/spreadsheets/d/1CerD7oQZVt0kYeSve9IGOtnIgJZ0MPLyTKwoKC77Dtc/edit?gid=0#gid=0)

We can have a 'sensitive string' data type, which will also log nothing in any production build