Add default value for internal tags

[swallez]

Adds an optional default configuration to internally tagged union definitions, e.g. @variants internal tag='type' default='object'

This is needed for:

• property mappings where object is the default (reported in elasticsearch-java#45)
• analyzers where custom is the default (reported in elasticsearch-java#216)

[delvedor]

LGTM

[github-actions]

The backport to 8.0 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-8.0 8.0
# Navigate to the new working tree
cd .worktrees/backport-8.0
# Create a new branch
git switch --create backport-1589-to-8.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 c61a7c773da33e1d9ff36a5d4a221dae21f43c30
# Push it to GitHub
git push --set-upstream origin backport-1589-to-8.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.0

Then, create a pull request where the base branch is 8.0 and the compare/head branch is backport-1589-to-8.0.