Consider adding peekn to GenericReader

Consider adding peekn to datacolumnreader. This would probably require modifying its underlying storage to a readonlymemory instead of span. Since we need to modify the data and return an owning copy in the case of big endian readers.

Also would be nice to have a peek of both strings, blobs and units.

Start by locating GenericReader and datacolumnreader, then inspect the underlying Span storage and the big endian reader behavior described in the issue. Define the scope for peekn and peeking strings, blobs, and units; done means the supported reader types can return the requested peeked data without the stated ownership problem.

Written by the indexing model from the issue text.