Add a migrations warning for ValueGeneratedOnAdd for a non-key property for SqlServer

[AndriySvyryd]

If a key property is configured as ValueGeneratedOnAdd without an explicit ValueGenerationStrategy, then it will fall back to the default ValueGenerationStrategy configured on the model, however it will have no effect on non-key properties, which might be confusing for users not closely familiar with these configuration options.
However it can still be a valid model if the user adds custom SQL to add a trigger or other mechanism to generate the value, therefore only a warning should be shown.

[divega]

EF Team Triage: We won't do this now but we can revisit if we get information from customers that indicates that the warning can help.

[divega]

If we ever do this we should consider generating a TODO comment.