Add plugin (user) version information

Add plugin (user) version information

This patch adds the BT_PLUGIN_VERSION_WITH_ID() and BT_PLUGIN_VERSION()
macros to the plugin development API (babeltrace/plugin/plugin-dev.h)
for the user to set a custom version (major, minor, patch, extra).

You can use the new bt_plugin_get_version() function to retrieve the
plugin's version, if set.

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