support flux-pmix extension for custom proc info

[garlick]

Problem: As noted in #109, some platforms may require Flux to set custom keys in the PMIx server to support application launch, but currently the only way to do that is to fork the flux-pmix shell plugin, which is likely not sustainable over time.

Idea: have the flux-pmix shell plugin call flux_shell_plugstack_call(3) for a new topic, say pmix.setprocinfo. A new shell plugin could then register a callback for that topic and set the custom key in its OUT arguments. After the plugstack call returns, flux-pmix would add any key/value pairs found in its OUT args to the PMIX_PROC_INFO_ARRAY.

[garlick]

Couple of further thoughts:

I'm remembering now that we are putting all the nspace keys into the proc info array (duplicating for every rank) even when the data is job or node scope. That is wrong and should eventually be fixed per #58. For this extension we probably should name the callback pmix.setnodeinfo (assuming the key you want to set is node scope) so when we fix that we will know where to put the data.

Also, since nspace data is typed, we should add the byte array object type to codec.c (with implicit base64 encode/decode) and decode values from plugin args without assuming any particular type. The new plugin would just need to know how to encode a byte object, e.g.

{"vendor.key":{"type":42,"data":"Zm9vIGJhcgo="}}

where type is the PMIX_BYTE_ARRAY constant value (eh I didn't look it up so 42) and data is the base64 encoded data.