zbus: optional unique channel numeric identifiers

Add the option for a unique numeric identifiers to be attached to a
zbus channel. This identifier can then be used to lookup the channel
structure at runtime. This is useful in two situations (that I can
immediately think of).

Firstly for external interaction, i.e a text shell or remote procedure
calls. The current state of a channel can be queried by an ID that never
changes, as opposed to the external entity needing to know the exact
memory address of the channel for a given application binary.

Secondly for integrating with dynamically loaded extensions (llext).
These extensions can hook into the existing data streams without each
individual channel needing to be exported and visible to the loader.

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