[Serializer] Add a MaxDepth handler

[javiereguiluz]

There's a new max_depth_handler config option, but as usual the pull request description is very poor and doesn't include any example: symfony/symfony#26108

[dunglas]

I'll handle this @javiereguiluz.
There is an usage example here anyway: https://github.com/symfony/symfony/pull/26108/files#diff-383be450d98e45fa294e0527b2e44640R635

[javiereguiluz]

Fixed by #9427.