Implement Boxplot aggregation

[russcam]

Relates: elastic/elasticsearch#51948

This commit implements the boxplot aggregation.
Integration tests run against XPackCluster because
it requires a license to use.

[russcam]

Will update this to run against ReadOnlyCluster, which uses the default distribution, so has a license applied.

[Mpdreamz]

LGTM once tests are happy 👍

[github-actions]

The backport to 7.7 failed:

The process 'git' failed with exit code 128

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-7.7 7.7
# Navigate to the new working tree
cd .worktrees/backport-7.7
# Create a new branch
git switch --create backport-4592-to-7.7
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 2f1528a3f29f8a88964dcae9d680229e898e7c45
# Push it to GitHub
git push --set-upstream origin backport-4592-to-7.7
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.7

Then, create a pull request where the base branch is 7.7 and the compare/head branch is backport-4592-to-7.7.

[github-actions]

The backport to 7.x failed:

The process 'git' failed with exit code 128

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-7.x 7.x
# Navigate to the new working tree
cd .worktrees/backport-7.x
# Create a new branch
git switch --create backport-4592-to-7.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 2f1528a3f29f8a88964dcae9d680229e898e7c45
# Push it to GitHub
git push --set-upstream origin backport-4592-to-7.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.x

Then, create a pull request where the base branch is 7.x and the compare/head branch is backport-4592-to-7.x.

[russcam]

manually ported to 7.x and 7.7