cpp-common/bt2: return `bt2c::CStringView` instead of `const char *`

cpp-common/bt2: return `bt2c::CStringView` instead of `const char *`

This patch makes any method in `cpp-common/bt2` return
`bt2c::CStringView` instead of `const char *` when possible to improve
the usability of the wrapping API.

In `bt2::CommonStringField` and `bt2::CommonStringValue`, I didn't
bother having a different raw value type to set vs. get, therefore the
value() getter returns `bt2c::CStringView` while the value() setter has
overloads for `bt2c::CStringView`, `const char *`, and
`const std::string&`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Iab7c7ea9a97035b9bea018f918a5d52516ab676c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11486
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>