lib: add field location API

lib: add field location API

Add API functions to create field location objects.

Field locations are created using bt_field_location_create.  The caller
must pass a trace class, which is used to verify that the MIP version
of the graph is >= 1.  The caller passes a (root) scope, and an array of
components, which are the names to get from the root of the scope to the
desired field.

Accessor functions are:

 - bt_field_location_get_scope
 - bt_field_location_get_component_count
 - bt_field_location_get_component_by_index

They are intended to be used by a component consuming a trace.

Philippe updated the documentation.

Change-Id: Iad554ce39e13194bd9cc056c2d6d43ba8b87effe
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7316
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7199