ES|QL: Test FORK with multi cluster

[ioanatia]

related #121950

We initially skipped CCS tests (PR) after we merged streaming support for FORK because the tests were flaky.

My assumption is that the fix from #127328 when we correctly wrapped all the sub plan executions in the same ComputeListener should also fix the failures we had seen with CCS when using FORK.
The failures we have seen with CCS had to do with the request returning incomplete results, where we did not include the results from some of the FORK branches. This seems to indicate we did not properly wait for all results to be returned, which should now be hopefully fixed with #127328.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[elasticsearchmachine]

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

[ioanatia]

got another failure after merging main - cannot replicate locally 😞

seems to be the same type of issue as before with results not coming back:

java.lang.AssertionError: Expected more data but no more entries found after [3] |  
-- | --
  | Data mismatch: |  
  |   |  
  | Actual: |  
  | _fork:keyword \| emp_no:integer \| x:keyword \| y:keyword \| z:keyword \| a:keyword |  
  | fork1         \| 10048          \| Florian   \| 10048     \| Syrotiuk  \| Florian 10048 Syrotiuk |  
  | fork1         \| 10081          \| Zhongwei  \| 10081     \| Rosen     \| Zhongwei 10081 Rosen |  
  | fork2         \| null           \| 2         \| 10081     \| 10048     \| null |  
  |   |  
  | Expected: |  
  | _fork:keyword \| emp_no:integer \| x:keyword \| y:keyword \| z:keyword \| a:keyword |  
  | fork1         \| 10048          \| Florian   \| 10048     \| Syrotiuk  \| Florian 10048 Syrotiuk |  
  | fork1         \| 10081          \| Zhongwei  \| 10081     \| Rosen     \| Zhongwei 10081 Rosen |  
  | fork2         \| null           \| 2         \| 10081     \| 10048     \| null |  
  | fork3         \| 10048          \| Syrotiuk  \| null      \| null      \| null |  
  | fork3         \| 10081          \| Rosen     \| null      \| null      \| null |  
  | fork4         \| 10048          \| abc       \| aaa       \| null      \| null |  
  | fork4         \| 10081          \| abc       \| aaa       \| null      \| null |  
  |  

putting this back in draft until I figure it out