[FrameworkBundle] Add parameter descriptors

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10345
| License       | MIT
| Doc PR        | n/a

I've added several descriptors for each format, henceforth :
``php app/console container:debug --parameter=database_name --format=txt`` will output:
``symfony``

``php app/console container:debug --parameter=database_name --format=json`` will output:
```json
{
    "database_name": "symfony"
}
```

``php app/console container:debug --parameter=database_name --format=xml`` will output :
```xml
<?xml version="1.0" encoding="UTF-8"?>
<parameter key="database_name">symfony</parameter>
```
and ``php app/console container:debug --parameter=database_name --format=md`` will output :
```md
database_name
`~~~~~~~~~~~~~
symfony
```

what do you think ?