bt2: move _append_option to non-const versions of variant classes

bt2: move _append_option to non-const versions of variant classes

_append_option should not be available on const versions of variant
classes.  Move it to the non-const versions.

Change-Id: I9b3b9ce2c3cfa629a00f95396b2136384d75d1fa
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2043
CI-Build: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>