Add as parameter to @Columns

[stephencelis]

This was allowed with @Selection before, so let's restore support for it.

[Playrom]

the PR works for non-optional tables

But the following declarations (which worked on previous versions) still do not works

@Columns(as: StructuredQueries.TableAlias<Child, Alias>?.self)
    let secondChild: Child?

Initializer 'init(father:firstChild:secondChild:)' requires the types 'Optional<TableAlias<Child, Alias>>.QueryValue' (aka 'Optional') and 'TableAlias<Child, Alias>?' be equivalent

[stephencelis]

@Playrom Thanks for the edge case repro! Should be fixed now.