cpp-common/bt2c: add CStringView equality and non-equality operators

cpp-common/bt2c: add CStringView equality and non-equality operators

Add `operator==` and `operator!=` functions to check equality or
non-equality between `CStringView` and another `CStringView`,
`const char *` and `std::string`.

Add a test to exercise those.

Change-Id: I2463966aae50792bb3e21ae65f9f124b08114499
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11681
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>