react-charting: Add controlled selection to Legends component

[insaneinside]

This PR adds controlled selection to the Legends component in @fluentui/react-charting.

With controlled selection, we allow the parent component to add additional integrations that interact with the selection state (for example, a button that selects all items, or a separate control that allows selecting grouped subsets of items).

Previous Behavior

Legends component allows setting initial selection via props, but does not allow fully-controlled selection behavior.

New Behavior

Legends component now supports fully-controlled selection behavior.

Related Issue(s)

None.