job-exec/sdexec: implement sd-bus based IPC to implement equivalent to FLUX_EXEC_PROTOCOL_FD

[chu11]

With the job-exec/sdexec implementation (PR #4070) there is no equivalent way to add the barrier added by #4155 to solve a number of corner cases.

At present, it seems impossible to solve it via a shared file descriptor like it was solved with the default exec job launch. A more likely solution would be to use sd-bus's IPC mechanisms. Have sdexec setup a "service" / "object" and have the job-shell communicate back to it.

Some notes for later:

key functions to research to implement:

sd_bus_request_name
sd_bus_add_object / sd_bus_add_object_vtable
sd_bus_add_match

random example i found online

https://cpp.hotexamples.com/site/file?hash=0xe0216674f5f8bdea4d4e0cadb97fe202b2a07e88e2ba55b6ae6db30fcd61bda7&fullName=openbmc-master/meta-phosphor/common/recipes-phosphor/obmc-phosphor-example-sdbus/obmc-phosphor-example-sdbus.c&project=bradbishop/openbmc