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 87.57764% with 40 lines in your changes missing coverage. Please review.

Project coverage is 77.90%. Comparing base (73cf23b) to head (12f7ebd).
Report is 4 commits behind head on main.

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	73.52%	4 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3038      +/-   ##
==========================================
- Coverage   77.94%   77.90%   -0.04%     
==========================================
  Files         242      243       +1     
  Lines       81739    82361     +622     
  Branches    81739    82361     +622     
==========================================
+ Hits        63711    64163     +452     
- Misses      14862    14992     +130     
- Partials     3166     3206      +40     

Flag	Coverage Δ
unittests	77.90% <87.57%> (-0.04%)	⬇️

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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#