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.

[insaneinside]

Team, thank you for your review! Please let me know if there are any additional changes that need to happen. Do I need to update any documentation, for example to add an example of a controlled legend selection to the React Charting demo site?

[AtishayMsft]

@insaneinside Could you add the scenario where this behavior is required in the PR description. We have a legends onChange handler callback when a legend value changes.

It would be great if you could add an example as well. You can add the example under react-examples/src/react-charting/Legends

[AtishayMsft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 4 pipeline(s).

[fabricteam]

📊 Bundle size report

✅ No changes found

[fabricteam]

🕵🏾‍♀️ visual regressions to review in the fluentuiv8 Visual Regression Report

react-charting-AreaChart 1 screenshots

Image Name	Diff(in Pixels)	Image Type
react-charting-AreaChart.Custom Accessibility.chromium.png	11	Changed

react-charting-LineChart 1 screenshots

Image Name	Diff(in Pixels)	Image Type
react-charting-LineChart.Gaps.chromium.png	1	Changed

[AtishayMsft]

There are some formatting and lint errors reported in the PR build.
https://uifabric.visualstudio.com/fabricpublic/_build/results?buildId=362233&view=logs&j=258ec178-2d8b-5611-7b9b-60c5c95dae55&t=cc94c182-d6f4-5dae-734d-3676c12cee2a

[AtishayMsft]

Could you add some margin between the example button and legend control in the example that you have added.
It is looking misaligned currently.