The scatter Rx offload capability is needed for the pmd_buffer_scatter
test suite. The command that retrieves the capability is:
show port <port_id> rx_offload capabilities

The command also retrieves a lot of other capabilities (RX_OFFLOAD_*)
which are all added into a Flag. The Flag members correspond to NIC
capability names so a convenience function that looks for the supported
Flags in a testpmd output is also added.

The NIC capability names (mentioned above) are copy-pasted from the
Flag. Dynamic addition of Enum members runs into problems with typing
(mypy doesn't know about the members) and documentation generation
(Sphinx doesn't know about the members).

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>

+    RX_OFFLOAD_VLAN_FILTER = 1 << 9
+    #: Device supports VLAN offload.
+    RX_OFFLOAD_VLAN_EXTEND = auto()
+    #: Device supports receiving segmented mbufs.
+    RX_OFFLOAD_SCATTER = 1 << 13