Today it's very difficult to see which indices are frozen or rather
throttled via the commonly used monitoring APIs. This change adds
a cell to the `_cat/indices` API to render if an index is `search.throttled`
Relates to #34352
This commit adds a rest endpoint for freezing and unfreezing an index.
Among other cleanups mainly fixing an issue accessing package private APIs
from a plugin that got caught by integration tests this change also adds
documentation for frozen indices.
Note: frozen indices are marked as `beta` and available as a basic feature.
Relates to #34352
David Turner
Simon Willnauer