Bluetooth: Mesh: Add Composition Data Page 1

Composition Data Page 1 contains information about relations between
models. Implementation saves relations into array of tuples
{ptr to base, ptr to extension, relation type} on model initialization
and uses this information to construct Composition Data Page 1.

Appropriate structures and methods were added to API, allowing user to
parse all received data into Composition Data Page 1 formats:
Composition Data Page 1 Element, Model Item, Model Extension Item.

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>