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 64.78%. Comparing base (e72b074) to head (adeb373).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18833      +/-   ##
============================================
+ Coverage     64.76%   64.78%   +0.01%     
  Complexity     1322     1322              
============================================
  Files          3393     3393              
  Lines        211025   211044      +19     
  Branches      33136    33140       +4     
============================================
+ Hits         136678   136728      +50     
+ Misses        63332    63295      -37     
- Partials      11015    11021       +6     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (?)
java-21	64.78% <100.00%> (+0.01%)	⬆️
temurin	64.78% <100.00%> (+0.01%)	⬆️
unittests	64.78% <100.00%> (+0.01%)	⬆️
unittests1	56.98% <100.00%> (+0.01%)	⬆️
unittests2	37.19% <25.00%> (+<0.01%)	⬆️

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[xiangfu0]

Found one high-signal correctness issue; see inline comment.

[xiangfu0]

This still does not round-trip the new metadata for legacy TIME columns. Schema.toJsonObject() serializes _timeFieldSpec through TimeFieldSpec.toJsonObject(), and that override never calls super.toJsonObject(), so fieldId / aliases are silently dropped for time columns even though this PR adds them generically on FieldSpec. Please thread these fields through TimeFieldSpec.toJsonObject() (and add a schema-level test) before merging.

[navina]

@xiangfu0 unrelated to my change, looks like tags,description and virtualColumnProvider are also not being serialized. Should I fix that in a separate PR?