Add 'dataset' size to cat indices and cat shards

[pquentin]

As was done in elastic/elasticsearch#98622

I'm making this change because this is a default column. In practice there are hundreds of other columns, but they're undocumented: elastic/elasticsearch#98955. I don't have current plans to add them.

[github-actions]

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

API	Status	Request	Response
cat.indices	🟢	33/33	32/32
cat.shards	🟢	11/11	11/11

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-2840-to-8.14
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 4cde24d9c2c273274ea3ac27ffe67c3687ea6d7a
# Push it to GitHub
git push --set-upstream origin backport-2840-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-2840-to-8.14.