[Validator] Backported #11410 to 2.3: Object initializers are called only once per object

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Before, object initializers were called multiple times if an object was validated in different groups in the same validation run. The initializers, however, are not aware of the current validation group, so calling them more than once does not make sense.

Now, object initializers are called exactly once per validated object.

See #11410