Variant Annotation columns count

Every so often, something goes wrong with the annotation (config etc), and columns are always blank.

If we kept stats per column on how many were non-null, we could find this pretty easily. It might also be useful to be able to estimate how a filter will behave.

Wondering whether to make a really big table (same as VariantAnnotation but with ints I guess - though that doesn't handle distinct values) or whether to just store it as JSON on `AnnotationRun`