zbus: add HEAP_MEM_POOL_ADD_SIZE_ZBUS

ZBus currently does not have a `HEAP_MEM_POOL_ADD_SIZE_ZBUS` when using
message subscribers or runtime observers, forcing the developer to set that
manually. Adding the configuration option to zbus would improve its
usability and make zbus easier to use. With this feature, zbus aligns with
the other Zephyr subsystems' heap memory allocation approach.

Signed-off-by: Rodrigo Peixoto <rodrigopex@gmail.com>