Implement alt var length view for external sort.

[lums658]

This PR implements an alternate view for var length data. Instead of maintaining an offsets range, as in the first var_length_view, this class materializes all of the individual subranges over the var length data. The associated unit tests for this PR include tests for sorting.

Advantages:

• The alt_var_length_view has the advantage of not returning a proxy when accessed and, as a result, can be directly sorted. Also, not having to create the proxy on access can result in more efficiency.
• The alt_var_length_view class can be constructed with either the "Arrow" format for offsets or the "TileDB" format. In the latter case, the constructor takes the offsets ranges, as well as the final value.

Disadvanatages:

• The alt_var_length_view requires more storage. An individual std::ranges::subrange consumes 16 bytes, whereas a size_t consumes 8 bytes. On the other hand, if sorting of the original var_length_view is of interest, the offsets range plus a permutation range will also result in 16 bytes storage per var length element.

Notes:

• The alt_var_length_view class ends up being a fairly thin wrapper around std::vector<std::ranges::subrange>>. However, it is considered not to be good practice to inherit from std::vector, so it was kept as member data.
• Because this view does maintain internal data that is not O(1), the copy constructor and copy assignment operator are disabled.

[sc-43636]

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TYPE: IMPROVEMENT
DESC: Implement alt var length view for external sort.