flowprobe: add support for reporting on inbound packets

flowprobe: add support for reporting on inbound packets

Type: feature

Currently, the plugin supports only IPFIX flow record generation for
outbound packets.

With this change:
 - add a new API message for enabling the feature on an interface that
   accepts direction (rx, tx, both);
 - update existing debug command for feature enabling to accept
   direction;
 - update existing debug command for showing currently enabled feature
   on interfaces to display direction;
 - update templates to include a direction field;
 - generate flow records on the specified direction and data path;
 - report direction in flow data;
 - update tests to use the new API;
 - add tests for inbound flows.

Change-Id: I121fd904b38408641036ebeea848df7a4e5e0b30
Signed-off-by: Alexander Chernavin <achernavin@netgate.com>