zbus: add user-configurable HEAP_MEM_POOL_ADD_SIZE_ZBUS

Zbus currently does only support default heap sizes.
The configuration option allows to decrease rom-footprint for embedded
applications. Adding the option to ZBus improves its flexibility and
while not increasing its complexity.

Add default of 0 value to compensate for previous cases where
HEAP_MEM_POOL_ADD_SIZE_ZBUS was defined as "".

Consistent naming 'Zbus'/'zbus' to 'ZBus'

Signed-off-by: Matthias Plöger <matthias.ploeger@phoenixcontact.com>