Table: customPlugins prop for user-authored plugins

Part of the Table feature suite tracker #3639.

**Feature area:** Extensibility

A Table prop accepting an array of user-authored plugins conforming to the public `TablePlugin` contract (`transformColumns`, `transformHeaderCell`, `transformBodyRow`, `transformBodyCell`, `transformScrollWrapper`, `transformTableContext`, `contextMenuActions`). Lets consumers extend the Table with their own transforms — no core change — alongside built-ins. Needs a documented/versioned contract, composition/ordering semantics vs. built-ins, and dev-time validation that a supplied plugin only uses supported hooks.

_Filed from the #3639 roadmap._