FORK: allow more commands in FORK branches

[ioanatia]

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)
• open up support such that we allow the rest of the commands in FORK branches
• add proper validation for the ones that we won't support (e.g. FORK within a FORK)

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Pinging @elastic/kibana-esql (ES|QL-ui)