Bluetooth: Mesh: Add support for Opcodes Aggregator models

Includes Opcodes Aggregator Server and Client. To use Opcodes Aggregator
client features with some client model, that client should support async
API.

Co-authored-by: Ludvig Samuelsen Jordet <ludvig.jordet@nordicsemi.no>
Co-authored-by: Mia Koen <mia.koen@nordicsemi.no>
Signed-off-by: Michał Narajowski <michal.narajowski@codecoup.pl>
Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>