Use signed integers in evpn gw API protobufs

We need to use signed integers in evpn gw protobufs as this is dictated by the [AIP rules](https://linter.aip.dev/141/forbidden-types).  Below you can see an example of integer that needs to be changed to signed.

https://github.com/opiproject/opi-api/blob/main/network/evpn-gw/l2_xpu_infra_mgr.proto#L139