feat: add a packed struct encoding to lance

[raunaks13]

Introduces a new PackedStruct encoding, should speed up random access for struct data, ref #2601

• Can currently support non-nullable, primitive fixed-length types (including fixed size list)
• Implemented as a physical type array encoder
• The user can select whether they want to use this encoding by specifying the field "packed" as true or false in the metadata. The default will use the old StructFieldEncoder

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.78744% with 34 lines in your changes missing coverage. Please review.

Project coverage is 79.25%. Comparing base (501a642) to head (825042a).

Files	Patch %	Lines
rust/lance-encoding/src/decoder.rs	82.02%	9 Missing and 7 partials :warning:
...e-encoding/src/encodings/physical/packed_struct.rs	96.61%	5 Missing and 2 partials :warning:
rust/lance-encoding-datafusion/src/zone.rs	0.00%	4 Missing :warning:
rust/lance-encoding/src/encoder.rs	93.75%	0 Missing and 3 partials :warning:
rust/lance-encoding-datafusion/src/lib.rs	0.00%	2 Missing :warning:
rust/lance-encoding/src/encodings/physical.rs	96.42%	1 Missing :warning:
rust/lance-encoding/src/encodings/utils.rs	92.85%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2593      +/-   ##
==========================================
+ Coverage   79.14%   79.25%   +0.11%     
==========================================
  Files         218      219       +1     
  Lines       63617    63927     +310     
  Branches    63617    63927     +310     
==========================================
+ Hits        50351    50668     +317     
+ Misses      10325    10311      -14     
- Partials     2941     2948       +7     

Flag	Coverage Δ
unittests	79.25% <91.78%> (+0.11%)	:arrow_up:

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