Add partial object sync specification for protocol version 6+

[JoaoDiasAbly]

Ticket: https://ably.atlassian.net/browse/AIT-236
Follow-up from: https://ably.atlassian.net/browse/AIT-127

[lawrence-forooghian]

Once the protocol is agreed it would be good if either @VeskeR or I could be added as a reviewer here too, since we'll be the ones implementing it 🙂

[JoaoDiasAbly]

Once the protocol is agreed it would be good if either @VeskeR or I could be added as a reviewer here too, since we'll be the ones implementing it 🙂

thanks for the heads-up! I will add you straight away if that's alright 🙏

[VeskeR]

Gonna add myself too for visibility