Add severity optional parameter to constraints

**Describe the solution you'd like**
In my use of pydeequ, I would like to be able to express the severity of a constraint(low, medium, high). The closest thing that exists to this currently is the CheckLevel, but that does not meet my requirements. I would like the user to be able to input severity as an optional parameter to a constraint, not the check object, so that this can be reflected in the VerificationResult dataframe.

**Describe alternatives you've considered**
I have tried adding severity from a higher level perspective, from a dataframe perspective, but that isn't granular enough. In my use, I am adding constraints on on check object and creating a VerificationResult dataframe from that. If this feature could be added to the constraints I would be really grateful. I am also open to suggestions.