Add ability to set custom routes as Resource children

[hiaselhans]

React admin makes it fast to develop admin-interfaces with react.
However, when resources get more and more functions it is a bit unhandy to pass CustomRoutes to

This PR adds children as a property of resources and makes it possible to define the whole resource with custom sub-routes in a seperate typescript module:

import ListView from "./list";
import CustomRoute from "./customroute";

const MyResource = () => {
    return {
        list: ListView,
        children: <CustomRoute/>
    }
}

[fzaninotto]

Thanks! Thats an interesting feature.

As it's an enhancement, please rebase your PR against the next branch.

Also, please document that feature and add some unit tests to make sure it works.

[WiXSL]

Thanks for your work!
You are still missing some test to prove that it works.

[hiaselhans]

thx @WiXSL !

i fixed the comments and added a test.
In my local env 3 unrelated tests are failing. However rebasing to next will flood the changeset in this PR

[fzaninotto]

Thanks!

[hiaselhans]

Great, thank you!