Add NEST support for EQL Search API

[stevejgordon]

Adds support for searching using EQL from the NEST client.

Contributes to #5584

@Mpdreamz - As this is a bit meaty, would you mind casting an eye over it? I opted not to create an interface for EqlSearchResponse and the types used for Event and Sequence which differs a little from the existing search code elsewhere.

[stevejgordon]

Gonna assume the license header failure is a line endings thing. Will review on Tuesday.

[Mpdreamz]

LGTM, the inferfaces for Search are there to facilitate mocking. This is the only API we support mocking to an extend so no interfaces for the other responses makes sense here too.

[github-actions]

The backport to master 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-master master
# Navigate to the new working tree
cd .worktrees/backport-master
# Create a new branch
git switch --create backport-5658-to-master
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 f96a6f94f82d81d954263df8e290c8b00ffefab1
# Push it to GitHub
git push --set-upstream origin backport-5658-to-master
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-master

Then, create a pull request where the base branch is master and the compare/head branch is backport-5658-to-master.