zbus: remove k_malloc dependency for `ZBUS_RUNTIME_OBSERVERS`

Remove the dependency on the system heap existing when enabling
`ZBUS_RUNTIME_OBSERVERS`. Instead the previously allocated memory is
required to be provided to `zbus_chan_add_obs` (which can still be
allocated through malloc).

Signed-off-by: Jordan Yates <jordan@embeint.com>
5 files changed