Allow value converters to change nullability of columns

[AndriySvyryd]

Fixes #24685

[ajcvickers]

@AndriySvyryd Despite what the issue title says, the only thing I was planning to do here was to allow a non-nullable CLR property to be configured as nullable in the database. Value converters that mess with nulls are problematic anyway, and just because a converter can convert nulls doesn't mean that it never results in a null. So the idea was that if you're sure (by whatever means necessary) that a nullable database column will not result in a null value in the CLR property, then we allow you to configure that. But we don't ever do that automatically.

That being said, if we want to do something more complex than that actually coupled to value converters, and we're confident it is correct, then I'm not necessarily against it.

[AndriySvyryd]

@ajcvickers Oh. That would be a part of #27970 then.

The approach in this PR works, but without #9329 there would be cases where setting a value converter wouldn't trigger conventions that depend on nullability. But the former solution somewhat suffers from this as well without #19806