cpp-common/bt2: do not use `bpstd::string_view`

cpp-common/bt2: do not use `bpstd::string_view`

Using `bpstd::string_view` in those contexts was a mistake: building a
`bpstd::string_view` object from a null-terminated string requires to
get its length (think strlen()), and an optional string view requires to
return `nonstd::optional<bpstd::string_view>` which means checking the
library pointer first.

All this means it's not a zero cost, and zero cost is what those C++
bindings aim to be.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib51fb9e326c4b4c450a381631b9db69f6cd0530b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11370
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>