Write ADR for protocol hints

When developing clients for ocis should return hints in responses if we think a request can be improved, e.g. when an upload is made without sending an `X-OC-Mtime` header. In this case we can return a header with a hint, similar to what the graph api does with `@microsoft.graph.hint` properties.

- [ ] What other hints can we add besides a missing upload mtime?
- [ ] What should we name the header?

Maybe related https://developer.mozilla.org/en-US/docs/Web/HTTP/Client_hints