GEP: Define Gateway API Interaction with Multicluster Services

[robscott]

What would you like to be added:
We need to define via documentation how Gateway API interacts with Multicluster Services. This will likely be a docs-only change, but may also include a change to API Spec to note "ServiceImport" is supported for "extended" conformance.

Why this is needed:
There are already implementations of Multi-Cluster Gateway either released or in progress, it would be great to ensure that we're all providing a consistent experience.

[robscott]

/kind gep
/assign
/cc @srampal

[costinm]

I assume this can also cover GAMMA ( both as backend reference, but also attaching to ServiceImport in addition to Service as a frontend).

[robscott]

I assume this can also cover GAMMA

Yep, that's the goal, hoping to discuss more at the GAMMA meeting tomorrow.

Shared this doc at today's community meeting: https://docs.google.com/document/d/1akwzBKtMKkkUV8tX-O7tPcI4BPMOLZ-gmS7Iz-7AOQE/edit. Will work on a follow up GEP once we get some feedback on the doc.