[Form] Add shortcuts to set data class and validation groups

Thanks to those simple shortcuts, when declaring forms as services, you can simply set the data class and/or validation groups.

Sample config:
```xml
<service id="acme.form.type.user" class="%acme.form.type.user.class%">
    <argument>%acme.model.user.class%</argument>
    <call method="setDataClass">
        <argument>%some_default_data_class%</argument>
    </call>
    <call method="setValidationGroups">
        <argument>%some_default_groups%</argument>
    </call>
    <tag name="form.type" alias="acme_user_form" />
</service>
```

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes*
| License       | MIT
| Doc PR        | n/a (yet)

\* - will fix if not =)