Tracked in #121652
At the moment FORK branches can only allow WHERE/LIMIT/SORT commands.

We want to remove this restriction, allowing FORK branches to support (almost) all commands.

One of the commands for which we know we will continue to have a restriction is the case of having a FORK commands inside another FORK branch.
This will be disallowed for the moment.

• FORK - allow EVAL/DISSECT/STATS in branches #125937 (just opens up support for DISSECT/STATS and EVAL)
• ES|QL: Fork - support GROK, CHANGE_POINT and COMPLETION in branches #129249
• FORK: Allow all processing commands #129465