string-format: introduce function to format component class name

string-format: introduce function to format component class name

The CLI has a function to print a component class name nicely,
print_plugin_comp_cls_opt.  The same functionality will be needed by
the Python bindings, move the function to a new convenience library.
Modify the function to return a string instead of printing directly to a
stream.  Update users accordingly.

Change-Id: I83fb61107ce93497ed34ebc383cbd3185c19752c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2432
Tested-by: jenkins <jenkins@lttng.org>