Gen shortcut for `COUNT(DISTINCT column_name)`

There's `Count` method that is generated for a table:
```go
Count(ctx context.Context, exec boil.ContextExecutor) (int64, error) // performs COUNT(*)
```

It'd be useful to have `CountDistinct` method:
```go
CountDistinct(ctx context.Context, exec boil.ContextExecutor, column string) (int64, error) // performs COUNT(DISTINCT column)
```