avatar driver get config name

Currently if one tries to create custom avatar driver in an extension, \phpbb\avatar\manager::get_driver_config_name() results in something like allow_avatar_vendor\avatardb\driver\db , which means the config value it would like to store in the database contains slashes, which isn't ideal.

the get_driver_config_name() method uses this regex to remove the driver name prefix ^phpbb\avatar\driver, but it should also replace all non-alphanumeric characters with spaces, or at least allow drivers to define their own custom name (something like get_name(), but used for config). Or at the very least string replace "\" with "_".

There is no real way to work around this as get_enabled_drivers() calls it as well, so even if the extension event cleaned up the config name, it still wouldn't enable the avatar driver.