doc/mesh-api: Add dictionary to model properties

doc/mesh-api: Add dictionary to model properties

This changes the signature of "Models" and "VendorModels" properties
on org.bluez.mesh.Element1 interface to contain a dictionary with
model options.

Models: signature change "aq" -> "a(qa{sv})"
VendorModels: signature change "a(qq)" -> "a(qqa{sv})"

Currently, the defined keywords for the options dictionary are
"Publish" - indicates whether the model supports publication mechanism.
            If not present, publication is enabled.
"Subscribe" - indicates whether the model supports subscription mechanism.
            If not present, subscriptions are enabled.

The dictionary allowed to be empty.

Change-Id: I7c4bd0f21c1a6c8c3d889b76433ec61ac6081598
Signed-off-by: anuj.bhumiya <anuj.bhumiya@samsung.com>