replace formOptions property by configureFormOptions method

[rgrassian]

Subject

I am targeting this branch, because BC.
The form options could allow some logic, for instance it could depends on the roles. It's not possible with a property, it will be with a method.

Changelog

### Added
- Added some `Sonata\AdminBundle\Admin\AbstractAdmin::configureFormOptions()` to do great stuff.

### Deprecated
- Deprecated `Sonata\AdminBundle\Admin\AbstractAdmin::formOptions` property. 

[OskarStark]

@VincentLanglet shall we introduce a new interface?

[VincentLanglet]

@VincentLanglet shall we introduce a new interface?

Why ? With which method ?

configureFormOptions is a protected method in the AbstractAdmin

[VincentLanglet]

I think the psalm error is not related to this PR but to the new symfony release

@sonata-project/contributors Does someone understand the message ? ^^'