GroupMod message should support selection_method and fields

[tnqn]

To implement DSR in Antrea (antrea-io/antrea#5025), we need to use hash selection method and specify fields when installing a group for a service with DSR enabled (See antrea-io/antrea#5025 (comment) for reason). But the library doesn't support these fields.

[tnqn]

@wenyingd I found selection_method is a Netronome extension, do you know where I can find how to construct a GroupMod message with these parameters?

[wenyingd]

I think we should firstly define a new struct for "Netronome selection_method" (https://github.com/openvswitch/ovs/blob/903294cde6e19b3eccefee13875f83b5ada2774c/include/openflow/netronome-ext.h#L36C8-L36C39), then append it into GroupMod.Properties (https://github.com/antrea-io/libOpenflow/blob/main/openflow15/group.go#L62)

A valid "Netronome selection_method" can be constructed reffering to https://github.com/openvswitch/ovs/blob/master/lib/ofp-group.c#L1210