report existing Mercury configuration in json if margo_init_ext() is used with an existing hg class and context

[carns]

The margo_init_ext() function allows a user to pass in a pre-configured hg_class and hg_context. In this case we should to retrieve the hg_init_info parameters from Mercury to report in the runtime margo json.

This will require an additional API hook in Mercury as well:

mercury-hpc/mercury#607

This is a relatively low priority issue but it would be nice to have.