store: `Append` allows non-adjacent ranges as input, while `heightSub` does not

There is a less common case possible now since adjacency broked. Append can now take non-contigious ranges of headers breaking the `heightSub`. [The logic fulilling requests](https://github.com/celestiaorg/go-header/blob/f512cad6b2b567003bf159887c865e27d943b25b/store/heightsub.go#L125-L137) in `Pub` method expects given header slice parameter to be contigious. If its not, a request either gonna be fulfilled with a wrong header or we panic with our of range exception