cpp-common/bt2: make `ConstComponentPorts::operator[](bt2c::CStringView)` return...

cpp-common/bt2: make `ConstComponentPorts::operator[](bt2c::CStringView)` return an `OptionalBorrowedObject`

The library functions that borrow ports by name return `NULL` when
no port with such name exist.  Change
`ConstComponentPorts::operator[](bt2c::CStringView)` to return an
`OptionalBorrowedObject` to model that in C++.

Change-Id: I2fafb509622e324980b88b70735e7beeb25f7fb4
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11802
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>