Use controller delegate callback for gathering the network credentials

[pan-apple]

Problem

As part of Rendezvous state machine, the controller sends network (e.g. WiFi) credentials to the device. Currently, the controller propagates a Rendezvous message from device to the controller app (e.g. CHIP Tool iOS app), which triggers the app to collect the credentials from the user. The app constructs another Rendezvous message containing the credentials and sends it to the device.

This puts extra burden on app to understand the Rendezvous message structure. Instead, the Rendezvous state machine should process the message, and call the delegate registered by app to collect the WiFi credentials. Then the state machine should construct the required Rendezvous message and send it to the device.

Proposed Solution

Update Rendezvous state machine, delegate interface and application code.

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