ethdev: add telemetry callbacks

ethdev: add telemetry callbacks

The ethdev library now registers commands with telemetry, and
implements the callback functions. These commands allow the list of
ethdev ports and the xstats and link status for a port to be queried.

An example using ethdev commands is shown below:

Connecting to /var/run/dpdk/rte/dpdk_telemetry.v2
{"version": "DPDK 20.05.0-rc0", "pid": 64379, "max_output_len": 16384}
--> /
{"/": ["/", "/ethdev/link_status", "/ethdev/list", "/ethdev/xstats", \
    "/help", "/info"]}
--> /ethdev/list
{"/ethdev/list": [0, 1, 2, 3]}
--> /ethdev/link_status,0
{"/ethdev/link_status": {"status": "UP", "speed": 10000, "duplex": \
    "full-duplex"}}
--> /ethdev/xstats,0
{"/ethdev/xstats": {"rx_good_packets": 0, "tx_good_packets": 0, \
    <snip>
    "tx_priority7_xon_to_xoff_packets": 0}}

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Ciara Power <ciara.power@intel.com>
Reviewed-by: Keith Wiles <keith.wiles@intel.com>