Add pivot panel and release 4.1.6

[petera2c]

Summary

• Adds a pivot panel in the column editor so people can put fields into Rows, Columns, and Values
• Multiple row fields now show as normal extra rows (one per combination), not expand/collapse groups
• Docs, changelog, and package version set to 4.1.6

Test plan

• Open the Pivot Panel Storybook example and place fields into Rows, Columns, and Values
• Confirm the table updates when Values has at least one measure, and goes flat again when Values is empty
• With Quarter + Product as Rows, confirm every combination is a full row (no chevrons / empty child column)
• Check aggregation dropdown and remove control look consistent with the rest of the side panel
• Skim /docs/pivot and changelog 4.1.6 for accuracy

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