Add RendezvousSession under src/transport

[vivien-apple]

Problem

Part of the rendezvous code lives into CHIPDeviceController.cpp and other parts into examples/wifi-echo/server/esp32/main/RendezvousSession.cpp

Some chunks of this code can be moved down into the stack in order to not pollute too much CHIPDeviceController and keep the Pairing mechanism unexposed to the application layer on the device.

Proposed Solution

Move some of the code into src/transport/RendezvousSession.cpp

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