[8.x] Add shard_failures to EQL response spec (#3372)

[luigidellaquila]

Manual backport of #3372

[github-actions]

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

API	Status	Request	Response
eql.delete	🟢	3/3	3/3
eql.get_status	🟢	3/3	3/3
eql.get	🟢	3/3	3/3
eql.search	🟢	38/38	38/38

You can validate these APIs yourself by using the make validate target.

[github-actions]

The backport to 8.18 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-8.18 8.18
# Navigate to the new working tree
cd .worktrees/backport-8.18
# Create a new branch
git switch --create backport-3674-to-8.18
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 f7d9aaf77666490664059f04c557c2790becb2e2
# Push it to GitHub
git push --set-upstream origin backport-3674-to-8.18
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.18

Then, create a pull request where the base branch is 8.18 and the compare/head branch is backport-3674-to-8.18.

[luigidellaquila]

No need to further backport, the change is already in 8.18