feat: add support for 'enum' extension type to v2 reader/writer

[westonpace]

The eventual goal is to get more compatibility with polars. There is still a bit of work missing (but this PR can probably stand on its own).

At the dataset level we should ensure that all files share the same dictionary within the dataset (we have some of this code so maybe it's just a matter of adding tests or ensuring myself it is still working)

A more tricky change is finding a way for polars to start using an extension array for the enum type. I'll try and open an issue and see if there is any interest in it.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.32335% with 39 lines in your changes missing coverage. Please review.

Project coverage is 78.38%. Comparing base (536e73d) to head (f0c02c4).

Files with missing lines	Patch %	Lines
rust/lance-arrow/src/dict_enum.rs	58.10%	27 Missing and 4 partials ⚠️
rust/lance-encoding/src/encoder.rs	82.35%	2 Missing and 4 partials ⚠️
.../lance-encoding/src/encodings/logical/primitive.rs	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3038    +/-   ##
========================================
  Coverage   78.38%   78.38%            
========================================
  Files         240      241     +1     
  Lines       77122    77287   +165     
  Branches    77122    77287   +165     
========================================
+ Hits        60449    60581   +132     
- Misses      13565    13597    +32     
- Partials     3108     3109     +1     

Flag	Coverage Δ
unittests	78.38% <88.32%> (+<0.01%)	⬆️

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[wjones127]

FWIW I notice this array type is considered "unstable" https://docs.pola.rs/api/python/dev/reference/api/polars.datatypes.Enum.html#