…non existent decorated services (mtarld)

This PR was merged into the 4.4 branch.

Discussion
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[DI] Add ability to choose behavior of decorations on non existent decorated services

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | #33522
| License       | MIT
| Doc PR        | symfony/symfony-docs#12442

# Handling decorations on non existent decorated services
Handle decorations on non existent decorated services by either throwing the service not found exception, silently ignoring services (decorator & decorated) all together or leave the decorated service to null (current behavior)

Something almost similar to how missing services as parameters are handles.

## Yaml configuration
```yaml
decorator:
    decorates: decorated
    decoration_on_invalid: ignore
```
Available values: `exception`, `ignore`, `null`. `exception` if nothing is specified.

## Xml configuration
```xml
<service id="decorator" decorates="decorated" decoration-on-invalid="ignore" />
```
Available values: `exception`, `ignore`, `null`. `exception` if nothing is specified.

## Behavior
- `exception`: Throws a `ServiceNotFoundException` telling that the decorator's dependency is missing
- `ignore`: Remove decorator definition. Decorator and decorated will not be available at all.
- `null`: Keep decorator but set decorated to null. Therefore, decorator `__construct` should be written with a nullable decorated dependency (`public function __contruct(?DecoratedInterface $decorated) {}`) and check should be done in other methods

Commits
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f167c77 Handle non existent decorated services

…ce" (mtarld)

This PR was merged into the 4.4 branch.

Discussion
----------

Documented "Handling decorations on non existent service"

Documentation part for the PR:
symfony/symfony#33854

Commits
-------

cb07e30 Add "Handling decorations on non existent service"