feat: initial support for parsing IF-MIB data (ADDON-39612)

- added a basic support for extracting from an SNMP WALK:
  - the total number of network interfaces,
  - all indexes associated to each network interface
  - all interface names available
 - some unit-test added

 In this way, we have the basic information required for
 sending additional data to Splunk.

splunk_connect_for_snmp_poller/manager/realtime/interface_mib.py

@@ -14,7 +14,7 @@
 # limitations under the License.
 #
 
-
+# See http://www.net-snmp.org/docs/mibs/interfaces.html for additional implementation details
 def extract_if_mib_only(translated_walk_result):
     return filter(
         lambda translation: all(