cpp-common/bt2: add asConst() methods

cpp-common/bt2: add asConst() methods

Add many asConst() methods to get the `bt2::ConstXyz` version of a
`bt2::Xyz` wrapper instead of using the constructor directly.

Just some sugar.

For example:

    void f(const bt2::Value val) {
        {
            const auto cVal = val.asConst();

            // Operate on `cVal`
        }
    }

I only added those methods where it was easy to do so.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Iabd9d6266fe75f33f2008c2320444624b842d9d2
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11232
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>