Fix eql.search response types

[pquentin]

Two changes were needed to pass validation:

• Make join_keys optional, as it's only present when using the by keyword. The first few examples in https://www.elastic.co/guide/en/elasticsearch/reference/8.14/eql.html don't have join keys.
• Add the optional missing boolean, as documented in https://www.elastic.co/guide/en/elasticsearch/reference/8.14/eql-syntax.html#eql-missing-events

Since PR validation is currently broken (see #2662), I validated those changes manually:

$ make validate api=eql.search type=response stack-version=8.15-SNAPSHOT
Validating endpoints                                                                                                                                          
Validating eql.search                                                          
eql.search response has been successfully validated!                                                                                                          
                                                                                                                                                              
✔ 32 out of 32 test response cases are passing.

I do not intend to merge this until PR validation passes.

[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	🟢	32/32	32/32

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

[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	🟢	32/32	32/32

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

[github-actions]

The backport to 8.14 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-8.14 8.14
# Navigate to the new working tree
cd .worktrees/backport-8.14
# Create a new branch
git switch --create backport-2663-to-8.14
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 10ce7ade3851f86259a11f339be28c399b1def9c
# Push it to GitHub
git push --set-upstream origin backport-2663-to-8.14
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.14

Then, create a pull request where the base branch is 8.14 and the compare/head branch is backport-2663-to-8.14.