Add in-pkts and out-pkts counters

[rgwilton]

WG Last Call: draft-ietf-netmod-intf-ext-yang-07: Vladimir issue 14.

14. I propose the in-pkts and out-pkts counters standardized too.
    https://github.com/YangModels/yang/blob/master/vendor/cisco/xe/1641/ietf-interfaces-ext.yang.
    And yes someone forgot to update the boilerplate text.

[samans]

Do not add counters, because this is the wrong module to do it in, it should be done as a new revision of ietf-interfaces.yang.

[rgwilton]

We should check with the NETMOD WG chairs where the best place would be to track this issue. There is an RFC 7223bis github project but I'm not really sure whether that is the right place to lodge an issue or not.