[Meta] TypeScript components that need to restrict their children

[chandlerprall]

Until microsoft/TypeScript#21699 is resolved it is not possible to restrict a children prop to a specific type/shape of React component. Use this ticket to track the places where children should be further restricted when it becomes possible.

• EuiGlobalToastListItem should only accept EuiToast children
• EuiValidatableControl's children must accept ref props and expose a setCustomValidity method

[cee-chen]

Seeing as it's been 4 years, Typescript is showing no movement on resolving this capability, and this is only a nice-to-have, we're closing this issue as not-planned.