[WIP][ExpressionLanguage] Use getters on protected/private properties

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | symfony/symfony-docs#3684

Allows for and defaults to accessing object properties through getters, if they exists.  

While this can be done with method calls as currently implemented, this is a convenience piece. Example would be when using Entities (ORM or ODM), which are typically created with private or protected properties and full getters/setters.

**todo**:
- [ ] Feeback
- [ ] Update tests