[FEAT] Add image mode casting

[Vince7778]

Adds the ability to cast between different image modes (at least, the ones that are supported). Also lets you perform this casting upon decode, which allows you to parse unsupported 16-bit images by converting them to 8-bit.

Closes #2495

[jaychia]

Looking good, let's get the Mode in docs and push this through

[codecov]

Codecov Report

Attention: Patch coverage is 80.59701% with 39 lines in your changes missing coverage. Please review.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2562   +/-   ##
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  Coverage        ?   63.37%           
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  Files           ?      982           
  Lines           ?   109358           
  Branches        ?        0           
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  Hits            ?    69308           
  Misses          ?    40050           
  Partials        ?        0           

Files	Coverage Δ
src/daft-core/src/python/series.rs	93.84% <100.00%> (ø)
src/daft-dsl/src/functions/image/mod.rs	100.00% <100.00%> (ø)
src/daft-dsl/src/python.rs	93.28% <100.00%> (ø)
src/daft-dsl/src/functions/image/decode.rs	57.50% <92.85%> (ø)
daft/series.py	89.69% <85.71%> (ø)
src/daft-core/src/series/ops/image.rs	74.35% <80.95%> (ø)
daft/expressions/expressions.py	93.18% <64.28%> (ø)
src/daft-core/src/array/ops/image.rs	91.48% <85.07%> (ø)
src/daft-dsl/src/functions/image/to_mode.rs	56.41% <56.41%> (ø)