Support reading tables with type widening in default engine

[johanl-db]

Type widening allows changing the type of an existing column in a Delta table to a wider type.
See https://github.com/delta-io/delta/blob/master/protocol_rfcs/type-widening.md

This change improves the default engine to enable upcasting conversions on read that are required to read tables that have a type change applied: data files written before the widening type change will contain data stored using a narrower type that must be upcast to the wider type in the current table schema.

Changes:

• Allow conversions:
  • int8 -> int16 -> int32 -> int64
  • float32 -> float64
  • date -> timestamp without timezone
  • integers -> float64
  • integers and decimals -> decimals, as long as the target precision and scale are sufficient to store all values without losing precision

This doesn't cover casting inside nested struct/array/map, which doesn't seem to be properly supported yet.

Testing:

• Added tests reading a table with various type changes applied, generated using delta spark 4.0 preview.
• Added unit tests for can_upcast_to_decimal, returning whether a given type can be upcast to a decimal.

[codecov]

Codecov Report

Attention: Patch coverage is 98.55072% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.09%. Comparing base (ed1919a) to head (9230d4b).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/engine/arrow_utils.rs	98.46%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #335      +/-   ##
==========================================
+ Coverage   73.86%   74.09%   +0.23%     
==========================================
  Files          43       43              
  Lines        8078     8154      +76     
  Branches     8078     8154      +76     
==========================================
+ Hits         5967     6042      +75     
- Misses       1732     1733       +1     
  Partials      379      379              

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[scovich]

Nicely surgical -- just extend the cast compat check and let the existing casting machinery do its job.

[nicklan]

lgtm with one nit. thanks!