sd-varlink: add sd_varlink_server_set_info

sd-varlink: add sd_varlink_server_set_info

Fixes https://github.com/systemd/systemd/issues/35508.

As reported in the bug, the values were hardcoded for the systemd project.
https://varlink.org/Service lists vendor, product, version, url, and interfaces
as the mandatory parameters, so add an interface to set the first four. The
last field is set automatically based on the registered interfaces as before.

If the values are not filled in, we return empty strings. With NULL,
'varlinkctl info' would say:
  (string):1:25: Object field 'vendor' has wrong type null, expected string.