Relax ChildAware API requirements

[CherryPerry]

Description

ChildAware API should allow any type instead of Node subtypes.
The current implementation is too permissive and does not allow to use interfaces as internode communication contract:

interface Child : Connectable<Input, Output> {
  sealed class Input
  sealed class Output
}

// We can't make ChildNode internal like we did in RIBs
class ChildNode(...): Node(...), Child by NodeConnector() {} 

// Somewhere in the parent node
whenChildrenAttached<Child>(...) // <-- Can't use Child interface here right now
whenChildrenAttached<ChildNode>(...) // <-- Have to use node instead with much wider public API

With this change it is possible to use Child interface to setup node connections, but still impossible to make ChildNode class internal. To do it we need to change Builder classes too, either introduce base Node interface like Rib interface or come up with something else.

Check list

• I have updated CHANGELOG.md if required.
• I have updated documentation if required.

[KovalevAndrey]

Maybe check if child is a Node to fail early? Rather that never get a callback?

[CherryPerry]

But it is not, that the main point. In my sample Child is just an interface, like we have in our templates.

[KovalevAndrey]

I don't mind, but we'll never know if we accidentally pass wrong class