Add C++ interface for the libbabeltrace2 `bt_field_path` API

Add C++ interface for the libbabeltrace2 `bt_field_path` API

This patch adds C++ wrappers for Babeltrace 2 field path objects.

The new available classes are:

* `bt2::ConstFieldPathItem`
* `bt2::ConstIndexFieldPathItem`
* `bt2::ConstFieldPath`

Those new classes follow the approach of other wrappers in
`src/cpp-common/bt2`.

You can't create a field path: it's what some field class methods (not
part of `src/cpp-common/bt2` yet) return.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I3414ac96ff2d305669ae24ef13aa8e88dad88df5
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4611