Consider marking relationship annotation as experimental for 9.1

[JamesNK]

After 9.0 branched I added an annotations and methods for modeling relationships: #5311. These are intented for the resource graph UI. However they're already used for References and Back references in the dashboard UI today.

The resource graph UI isn't planned for 9.1 which is the main use case for relationships. I think we should consider marking relationships as an experimental API so we can change it if required when the graph ships.

[davidfowl]

Can you include the apis you are considering?