Add optional fieldId and aliases to FieldSpec for column identity

[navina]

What

Adds two optional, nullable metadata fields to FieldSpec (pinot-spi):

Field	Type	Meaning
fieldId	Integer	A stable, name-independent identifier for the column.
aliases	List<String>	Alternate / historical names for the column.

Both follow the existing description / tags convention on FieldSpec:

• fieldId is serialized with @JsonInclude(NON_NULL), aliases with @JsonInclude(NON_EMPTY).
• Both are mirrored in the manual toJsonObject() serialization and included in equals() / hashCode().
• They are not part of schema backward-compatibility validation, so adding or changing them doesn't fail a schema update today.

Why

fieldId provides a stable column identity that is decoupled from the column name. Together with aliases, it lays the groundwork for advanced schema evolutions like column rename in Pinot — a rename can preserve the column's identity and record the prior name as an alias, instead of looking like a drop-plus-add.

Compatibility

• No impact on existing tables. Both fields default to null/empty and are omitted from JSON, so the serialized schema and wire format are unchanged for any column that doesn't set them.
• Mixed-version safe. Older components ignore the unknown JSON properties on read; newer components emit nothing for columns without these fields. They are excluded from backward-compatibility checks, so a schema carrying them validates against one that doesn't.
• No updates to query/ingestion paths or segment metadata today — this change only adds the fields to the schema model.

Tests

FieldSpecTest adds coverage for the new getters/setters, JSON round-trip (including omission when unset), and equals/hashCode participation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.81%. Comparing base (52b8fdc) to head (7a6309a).

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18833      +/-   ##
============================================
+ Coverage     64.78%   64.81%   +0.02%     
  Complexity     1319     1319              
============================================
  Files          3392     3392              
  Lines        210988   211004      +16     
  Branches      33127    33132       +5     
============================================
+ Hits         136687   136758      +71     
+ Misses        63283    63206      -77     
- Partials      11018    11040      +22     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-21	64.81% <100.00%> (+0.02%)	⬆️
temurin	64.81% <100.00%> (+0.02%)	⬆️
unittests	64.81% <100.00%> (+0.02%)	⬆️
unittests1	56.96% <100.00%> (-0.04%)	⬇️
unittests2	37.23% <11.76%> (+0.05%)	⬆️

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