Add spec for EQL allow_partial_search_results

[luigidellaquila]

Add spec for EQL allow_partial_search_results and for allow_partial_search_results parameters.

These allow EQL queries to keep running in case of shard failures.

Relates to elastic/elasticsearch#116388

[github-actions]

Following you can find the validation results for the API you have changed.

API	Status	Request	Response
eql.search	🟢	32/32	32/32

You can validate this API yourself by using the make validate target.

[flobernd]

LGTM! Thank you 🙂

[github-actions]

The backport to 8.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.x 8.x
# Navigate to the new working tree
cd .worktrees/backport-8.x
# Create a new branch
git switch --create backport-3342-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 5987f79aed121f3657fc75769e494b26404d512b
# Push it to GitHub
git push --set-upstream origin backport-3342-to-8.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.x

Then, create a pull request where the base branch is 8.x and the compare/head branch is backport-3342-to-8.x.