Add support for plugin-defined custom components in union types

[swallez]

Add support for custom variants in types representing components that can be extended with plugins in Elasticsearch, such as queries, aggregations, property mapping, etc. These types now have an additional _custom variant that accepts a name (the plugin-defined extension identifier) and an arbitrary value.

Fixes #252.

Doc for this feature is added in PR #371

[github-actions]

The backport to 7.17 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-7.17 7.17
# Navigate to the new working tree
cd .worktrees/backport-7.17
# Create a new branch
git switch --create backport-370-to-7.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 adf11b54dc93cb26e6b4092379e0fdb30a593029
# Push it to GitHub
git push --set-upstream origin backport-370-to-7.17
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.17

Then, create a pull request where the base branch is 7.17 and the compare/head branch is backport-370-to-7.17.