Reduce redundancy for `RootProperty` validation

The validation of each [`rootProperty`](https://github.com/CesiumGS/3d-tiles/blob/21aba4796c8281f33e3235493ae9040b70a50bc8/specification/schema/common/rootProperty.schema.json) currently starts with a common block:

- Make sure that the given value is an object
- Validate the object as a RootProperty: Use the `RootPropertyValidator` to check that `extensions` and `extras` have a proper JSON structure)
- Perform the validation of the object in view of the extensions that it may contain: Use the `ExtendedObjectsValidators` to check whether the object contains a known extension object, and whether a special `Validator` was registered for that case
- If there was an extension validator that overrides the default validation, then skip the remaining validation.

It might also be worth considering to model the validation process more generally and explicitly as some sort of "chain", roughly as in the pseudocode

let result = true;
for (const step of chain) {
const stepResult = step.apply(input);
if (!stepResult.valid) result = false;
if (!stepResult.continueValidation) break;
}

The redundancy for now is not so high that it would warrant larger engineering efforts. But eventually, this could lead to a more flexible (and extensible) validation process in general, so should be considered at some point in time.

Start with the rootProperty validation flow and the RootPropertyValidator and ExtendedObjectsValidators mentioned in the issue. Trace how each rootProperty repeats the object, RootProperty, extension, and override checks. Done means the common validation is centralized or modeled consistently without changing existing validation behavior; consider the proposed chain only if it fits the scope.

Written by the indexing model from the issue text.