feat: finish up variable-length encodings in the full-zip path

[westonpace]

This adds the last structural path for 2.1, full zip encoding of variable length data. Scheduling this turned out to be a little trickier than I had planned. There is no easy way to know where to slice the fully-zipped buffer when doing decoding. Currently we settle this problem by unzipping in the indirect scheduling task. There are some alternative possibilities that I have documented but for now I think this will be good enough and we can iterate on this going forwards.

[westonpace]

Leaving in draft until #3335 merges

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 71 lines in your changes missing coverage. Please review.

Project coverage is 78.73%. Comparing base (4cb37c1) to head (82dd08c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../lance-encoding/src/encodings/logical/primitive.rs	90.09%	26 Missing and 7 partials ⚠️
rust/lance-encoding/src/encodings/physical/fsst.rs	60.52%	13 Missing and 2 partials ⚠️
rust/lance-encoding/src/data.rs	0.00%	9 Missing ⚠️
rust/lance-file/src/v2/reader.rs	0.00%	7 Missing ⚠️
rust/lance-encoding/src/encoder.rs	85.00%	3 Missing ⚠️
rust/lance-encoding/src/decoder.rs	85.71%	2 Missing ⚠️
rust/lance-encoding/src/encodings/physical.rs	0.00%	1 Missing ⚠️
...st/lance-encoding/src/encodings/physical/binary.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3344      +/-   ##
==========================================
+ Coverage   78.68%   78.73%   +0.05%     
==========================================
  Files         250      250              
  Lines       90734    91243     +509     
  Branches    90734    91243     +509     
==========================================
+ Hits        71391    71842     +451     
- Misses      16406    16453      +47     
- Partials     2937     2948      +11     

Flag	Coverage Δ
unittests	78.73% <85.71%> (+0.05%)	⬆️

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