Add bt2c::join()

Add bt2c::join()

This new function template joins the strings of some container with
some delimiter.

The container needs a forward iterator and its elements need the data()
and size() methods.

The container may be empty.

For example:

    int main()
    {
        std::cout << bt2c::join(std::vector<std::string> {
            "salut", "meow", "mix"
        }, ", ") << '\n';
    }

will print

    salut, meow, mix

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Id3ff90cf3b2fa4336b71860dbe3c0dff83668607
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12735