src.ctf.lttng-live: make live_viewer_connection::{relay_hostname,target_hostname...

src.ctf.lttng-live: make live_viewer_connection::{relay_hostname,target_hostname,session_name,proto} GStringUPs

These values come from GStrings from the common code, not yet C++-ified,
so it's simpler for now to use GStringUP to automatically manage the
GString lifetimes locally.  If the common code ever becomes C++, it can
all be replaced with std::string.

Change-Id: I913ab51488fc934cd049e530a2b7583cc1cefff6
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8470
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>