[7.x] Add multi field info to the IndexPattern (#33681)

[Bargs]

Adds two fields to the IndexPattern Field:

• parent - the name of the field this field is a child of
• subType - The type of child this field is. Currently the only valid value is multi but we could expand this to include aliases, object children, and nested children.

The thinking behind implementing these two new properties instead of a simple isMultiField flag is that it should be generic enough to describe other sorts of parent -> child relationships between fields.

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[elasticmachine]

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