Allowing custom reconciler host-context in React 18

in a custom renderer we would like to do something like this: ```jsx <inject foo="bar"> <group> <group> <obj /> </group> </group> </inject> ``` ✅ `obj` is supposed to know it is within `inject`, this is possible ❌ `inject` is supposed to make its props (foo="bar") the host context for all of its nested contents, `obj` should be able to access it. this is not possible we create a host context like this: ```jsx getRootHostContext(rootContainer) { return {} } getChildHostContext(parentHostContext, type, rootContainer) { if (type === "inject") {} else return parentHostContext ``` when `getChildHostContext` gets called for `inject`, we only have access to string type, not props. i thought i can return an empty object `{}` and fill it later in: ```jsx finalizeInitialChildren(instance, type, props, rootContainer, hostContext) { if (type === "inject) hostContext.props = props ``` but when `finalizeInitialChildren` is called for `inject` it refers to root context, it can't refer to its own host context that was created by `getChildHostContext`. i find this puzzling, we don't seem to have any relation between a host context and the element that created it. ## solution if we can access the current context it would be very useful to create a richer host context, that will also update. ```jsx finalizeInitialChildren(instance, type, props, rootContainer, hostContext, currentContext) prepareUpdate(instance, type, oldProps, newProps, rootContainer, hostContext, currentContext) ``` alternatively, allowing us to receive props would at least allow for a static host context that doesn't update but at least carrier some deeper information. ```jsx getChildHostContext(parentHostContext, type, rootContainer, props) ```