[BUG] Mark APIs experimental vs. stable

[saimedhi]

What is the bug?

The OpenSearch API specification lacks a parameter or key to indicate the stability status of an endpoint (whether it's experimental or stable)

What is the expected behavior?

A parameter or key to indicate the stability status of an endpoint (whether it's experimental or stable)

[nhtruong]

We should add x-experimental: true extension.

[Xtansia]

This potentially should be an enum of x-stability: (stable|experimental) (with stable being assumed if not present). As we might have multiple levels of stability present within the server. From elastic I believe it was stable, beta, experimental or something similar, so not sure if that scheme is still in use by OpenSearch core.

[dblock]

All APIs are considered stable unless marked experimental in OpenSearch, we should match that and only add x-experimental.