[flagd] use new protos

[toddbaert]

With the addition of the new protos (flagd.evaluation.v1, flagd.sync.v1), we must update all clients (flagd providers) to use these endpoints.

We use the buf-generated code already; we mus simply update imports like this one to use the new artifacts.

new protos in buf: https://buf.build/open-feature/flagd
new proto source: https://github.com/open-feature/flagd-schemas/tree/main/protobuf/flagd 

[toddbaert]

Complete with #451