Bluetooth mesh message context API needs to expose DST address of RX messages

bt_mesh_msg_ctx contains a field, addr which is the remote address of the sending device i.e. the SRC field. It does not currently include the destination address (DST) set by the sender.

The mesh models spec describes various rules for handling identical transactions. For example in 3.3.2.2.2 it says "...it shall set the Generic Level state to the Level field of the message, unless the message has the same values for the SRC, DST, and TID fields as the previous message received within the last 6 seconds". Note that the condition includes matching on the DST value.

It's possible for a device to change the DST address it is using. For example, you can imagine a rotary wall dimmer with a "zone" selector. The selector may have two positions: 1 - group A and 2 - group B. The selector affects what DST the dimmer uses when sending Delta Set messages.

Summary: please make the DST field from RX messages available in the API, presumably by adding it to bt_mesh_msg_ctx.

Thanks