Add bt2::Common{Field,FieldClass,Value,Message}<>::as<>()

Add bt2::Common{Field,FieldClass,Value,Message}<>::as<>()

Those new as() method templates conceptually cast their object to the
first template parameter. Those are shorthands to use specific types
within a template function, for example:

    template <typename FieldT, typename ValT>
    void g(bt2::Field field, const ValT val)
    {
        // ...
        field.as<FieldT>() = val;
        // ...
    }

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ifc1f49487ac6ef9c143bf4c2faa3373f4eaccd9e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8099
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10803
Tested-by: jenkins <jenkins@lttng.org>