Add Nested Row 's field_id

[loserwang1024]

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• I searched in the issues and found nothing similar.

Description

Purpose

Linked issue: close #2310 , add field id to nested rows.

Brief change log

All fields are assigned unique, sequential IDs in a flattened order, regardless of nesting level. For example:

struct< 
  a: tinyint, 
  b: struct< 
    c: tinyint, 
    d: struct< 
      e: tinyint, 
      f: tinyint
    >, 
    g: string 
  > 
>

Then the field Id for each field is:

Field Name	Field ID
a	0
b	1
b.c	2
b.d	3
b.d.e	4
b.d.f	5
b.g	6

Why Flatten Numerical Order?

• Simplifies ID Management: No need to compute hierarchical offsets (e.g., parent_id * depth + child_index).
• Compatibility: Works seamlessly with flat data structures like Arrow's RecordBatch. In our FileLogProjection, same thing is also done. Thus later projection push down will be more easier.

API and Format

Add field_id to org.apache.fluss.types.DataField.

Documentation

Willingness to contribute

• I'm willing to submit a PR!