Expose Subscription liveness callbacks to the application layer

[sagar-apple]

Problem

There doesn't seem to be a way for an application to sign up for SubscriptionTerminated notifications from the SDK.

This means that if a subscription expires, no notification is sent to the application to attempt a retry, for example.

Proposed Solution

Expose the IMDelegate APIs to the application layer.

[bzbarsky-apple]

@mrjerryjohns @yunhanw-google

[mrjerryjohns]

I presume you're referring to the client side here.

There are a couple of different callback APIs in question here - starting at the base with ReadClient::Callback, terminal errors are delivered through ReadClient::Callback::OnError, while non-terminal, attribute-path specific errors are delivered through ReadClient::Callabck::OnAttributeData.

The next level up is chip::Controller::TypedReadCallback (which is passed into chip::Controller::ReadAttribute). The OnErrorCallbackType signature will convey terminal errors if aPath == nullptr, including those that are fatal to the subscription, with the actual error embedded in a combination of the IM status code + CHIP_ERROR provided.

What we don't do well today is to convey liveness timeout failures correctly with a precise error code - it's conveyed as a CHIP_ERROR_TIMEOUT when we should in fact, be sending something more descriptive.

Levels above that are still WIP I believe..

[mrjerryjohns]

@sagar-apple is this good enough for you?

[mrjerryjohns]

@yunhanw-google to fix the emission of a more precise error code for liveness failure.

[bzbarsky-apple]

@sagar-apple needs those "levels above" fixed is the point.

[yunhanw-google]

Per chat with @bzbarsky-apple , Boris's incoming PR would take care of this, thanks