lib: add bt_trace_class_get_graph_mip_version

lib: add bt_trace_class_get_graph_mip_version

Add bt_trace_class_get_graph_mip_version, which returns the MIP version
for the graph the trace class is in.

This will be necessary for the Python bindings functions dealing with
trace IR that need to change their behavior according to the MIP
version.  Right now, the MIP version can be obtained from a self
component.  But some Python functions (for instance
_TraceClass.create_variant_field_class) only have a IR object to work
with, and it's not possible to get the self component from there.
Adding a function to get the self component from a trace class was
considered, but it was deemed to complex because of object lifetime and
refcount cycle issues.

Philippe updated the documentation.

Change-Id: Ie31be7d31baf1242bad3927021de7c949f281247
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10155
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12719