Add bt_component_class_freeze()

Add bt_component_class_freeze()

It is somewhat essential for a component class provider (plugin or
whatever else) to make sure that its created, shared component class is
not modified once properly configured.

The new bt_component_class_freeze() function freezes a component class
object so that it becomes immutable.

bt_component_create() also has the side effect of freezing its class.
This is analogous to bt_event_create() which freezes its class.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>