CmsPage component in SPA React SDK should accept children as a prop

The README in the SPA React SDK states:

Please note that `<CmsPage>` takes children as a function instead of as a prop, because it is not possible to provide state through context providers due to a bug in React currently. So make sure to wrap any children of `<CmsPage>` in a function.

It's unclear which "bug" is being referred to. Having to use a 'render function' to supply the children of CmsPage is odd, and a quick POC suggests that passing the children as a prop is perfectly well possible. We should start supporting this.

Can be implemented backwards compatible: CmsPage can check whether props.children is a function (and then invoke it) or not (and this use it as-is).