Need a shared way to preview experimental API and Iceberg REST semantics

## Summary

Gravitino should offer **early access** to upcoming user-facing semantics—
Iceberg REST and similar work in other projects—while those specs are
still moving. Today that means either forcing new behavior on every
Spark/Trino client or inventing a one-off switch.

## Desired outcome

A shared preview path: environments can offer it, callers opt in and
accept instability, everyone else stays on the stable contract, and a
preview can freeze at a release and line up with Iceberg or other
upstream work when it lands.

## Acceptance criteria

- This issue is accepted and work is underway.
- Design is reviewed and agreed.
- Preview control-plane functionality is implemented.
- This issue is closed.

## Scope and boundaries

- **In**: preview and graduation control plane for Gravitino API and Iceberg REST.
- **Out**: per-feature models, authz, forking Iceberg, OpenFeature, parallel experimental routes.

## Related work

Related to: https://github.com/apache/gravitino/pull/12471

Nevin
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