API nit: standardize plurality of enums

[mcginty]

Looking through the codebase I see a couple examples of enums where some are plural (FooAttrs) and some aren't (FooAttr):

rg 'enum .*Attr'

Might be good to settle on one in future breaking change updates (it looks like most projects choose the plural version).

[little-dude]

This is a good idea in theory, but in practice this is a breaking change which will force users to update their code for no gain in terms of features or bug fixing. For the record:

❯ rg 'enum .*Attr'
netlink-packet-generic/src/ctrl/nlas/mcast.rs
15:pub enum McastGrpAttrs {

ethtool/src/link_mode/attr.rs
57:pub enum EthtoolLinkModeAttr {

ethtool/src/coalesce/attr.rs
40:pub enum EthtoolCoalesceAttr {

netlink-packet-generic/src/ctrl/nlas/oppolicy.rs
54:pub enum OppolicyIndexAttr {

netlink-packet-generic/src/ctrl/nlas/policy.rs
99:pub enum NlPolicyTypeAttrs {

ethtool/src/ring/attr.rs
26:pub enum EthtoolRingAttr {

netlink-packet-generic/src/ctrl/nlas/mod.rs
25:pub enum GenlCtrlAttrs {

netlink-packet-generic/src/ctrl/nlas/ops.rs
15:pub enum OpAttrs {

ethtool/src/message.rs
59:pub enum EthtoolAttr {

ethtool/src/pause/attr.rs
25:pub enum EthtoolPauseStatAttr {
71:pub enum EthtoolPauseAttr {

ethtool/src/feature/attr.rs
55:pub enum EthtoolFeatureAttr {

netlink-packet-wireguard/src/nlas/device.rs
16:pub enum WgDeviceAttrs {

netlink-packet-wireguard/src/nlas/peer.rs
83:pub enum WgPeerAttrs {

netlink-packet-wireguard/src/nlas/allowedip.rs
16:pub enum WgAllowedIpAttrs {

[little-dude]

ethtool seems to use the singular version consistently. Changing all the type names may be a bit annoying for users, but we could probably do that incrementally: change the names, but keep aliases with a deprecation warning. netlink-generic-packet uses Attrs except for OppolicyIndexAttr. I think we could just change this one. netlink-packet-wireguard uses the plural version consistently.