bt2: Add `*ValueConst` classes and adapt tests

bt2: Add `*ValueConst` classes and adapt tests

Split Python Value classes to mimic the type safety offered by the C
api. Const classes offer a read-only view of the data. Non-Const classes
subclass their respective Const classes.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I479db305a281063c7ee8aee2890c33c0b22e7153
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1983
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>