Rabbit resources jobspec update

[jameshcorbett]

The dws-jobtap plugin grabs a reference to the user's jobspec and modifies it in an RPC callback.

The flux-core jobtap interface exposes a function flux_jobtap_jobspec_update_pack (flux_plugin_t *p, const char *fmt, ...). The dws-jobtap plugin should use that function (since it is part of the official interface) rather than relying on manipulation of the copy of jobspec held by the job manager. However, the function does not accept a jobid argument, which it would need to do in order to be used properly by dws-jobtap, since dws-jobtap does not modify the jobspec from within a jobtap callback.