Make the choice to dictionary-encode data results explicit

[jleibs]

What

• When requesting a Column explicitly, you can now use the new ColumnDescriptor::DictionaryEncoded(descriptor) to indicate that you want a dictionary-encoded view of a column.
• Components that are neither part of the chunk, nor dictionary encoded, result in row-slices instead.
• If there are no row-slices, we preserve the old full-chunk based join mechanism, giving large record-batches.
• If there are row-slices, you get single rows.

Future work:

• Dataframe API should expose mechanism to return sorted results. #7414

Checklist

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• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

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