fix(cdk/tree): implement typeahead label

[zarend]

Implement typeahead labels for Tree. Follow the established pattern that Menu and List use.

• implement CdkTreeNode#cdkTreeNodeTypeaheadLabel
• implement CdkTreeNode#getLabel

[github-actions[bot]]

Deployed dev-app for e90ed304cfa828ce76f389473caa19ff9e39fcb1 to: https://ng-dev-previews-comp--pr-angular-components-28251-dev-xww57l2z.web.app

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[crisbeto]

Closing since this was merged as a part of #29055.

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