jobtap: allow plugins to call other plugin callbacks

[grondo]

A use case has come up where it may be useful to allow a plugin to call other plugins in the jobtap plugin stack, e.g. similar to flux_shell_plugstack_call(3).

However, this could be slightly awkward since the jobtap interface is job focused (which is probably why we didn't think of this before). Likely something with a prototype like this would work:

int flux_jobtap_call (flux_plugin_t *p, const char *topic, flux_plugin_arg_t *args);