eosfs: don't rely on systemcall to obtain user unix IDs/GroupIds

[labkode]

In the EOS filesystem implementation we rely on using systemcalls to obtain the user uid/gid. This implies having local users where reva runs and that sucks:
https://github.com/cs3org/reva/blob/master/pkg/storage/utils/eosfs/eosfs.go#L1219

We should obtain this information directly by calling the UserProviderAPI (through the Gateway).

[butonic]

we could mint the uid and gid into the token. In any case, we need to add uid and gid to the cs3 User: https://cs3org.github.io/cs3apis/#cs3%2fidentity%2fuser%2fv1beta1%2fresources.proto

[labkode]

@butonic that is a nice idea. The uid/gid are quite specific for the EOS filesystem so I think they should still be part of the Opaque part unless there are other fs that will needed and then we can make it first class citizen. Any thoughts?