SdcPropKeysConstraintValidator::validate() should complain about extraneous keys too, not just missing keys

None of the code in 0.x can generate this, it had to be manually constructed by @tedbow in a test.

Therefore, even sites created on beta1 should never run into this. Even if that's the case, the update path would be trivial 👍

I updated the SdcPropKeysConstraintValidator::validate() method to check for extra keys in the input mapping that are not defined in the SDC props.
To support this, I added a new extraMessage property to the SdcPropKeysConstraint class for the error message.

Looking good! That’s step 1 done 😄

Steps 2 and 3 are in the issue summary, do you think you could handle those too? 🙏