Improve toString()

[swallez]

Improve the toString() method on API objects:

• the class name is prepended to the JSON representation of the class, to allow identifying it, and also avoir misuse of toString() as a way to serialize objects to JSON
• add static setter and getter in replacement of the (now deprecated) JsonpUtils.MAX_TO_STRING_LENGTH
• implement toString() for tagged unions (i.e. variant) objects

[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-320-to-7.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 bd237d14210508fca4def6f28e34231055aed531
# Push it to GitHub
git push --set-upstream origin backport-320-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-320-to-7.17.