Allow #[diesel(treat_none_as_{null, default_value} = …] on fields

[moulins]

If present on a field, this will override the corresponding container-level attribute.

This restores feature parity with the old diesel 1.x versions, which allowed these two attributes on struct fields.

[pksunkara]

Need tests, both happy and error cases.

[moulins]

I've added/modified tests to exercise field-level #treat_none_as_null, and to reject invalid embed, treat_none_as_default_value combinations.

However, I didn't see any pre-existing tests for treat_none_as_default_value to modify and I'm not sure how it should be tested; what should be done here?

[moulins]

Something I just thought about: as implemented, putting an explicit treat_none_as_null on a non-Option field will do nothing; maybe it should be an error instead?

[Ten0]

maybe it should be an error instead

yes that makes sense, that would be better

[weiznich]

I've rebased this branch to pull in the changes from #3729 to fix the broken tests.