[Yaml] Allow to get all the enum cases

[phansys]

Q	A
Branch?	7.1
Bug fix?	no
New feature?	yes
Deprecations?	no
Issues	n/a
License	MIT

With this addition, the !php/enum syntax is allowed to expose an array with all the enum cases (the result from UnitEnum::cases()). This is useful for cases like choices option from the Choice validation constraint:

BEFORE:

App\Entity\User:
    properties:
        status:
            - Choice:
                choices:
                    - !php/enum 'App\Entity\Enum\UserStatus::Enabled'
                    - !php/enum 'App\Entity\Enum\UserStatus::Disabled'
                    - !php/enum 'App\Entity\Enum\UserStatus::Blocked'

AFTER:

App\Entity\User:
    properties:
        status:
            - Choice:
                choices: !php/enum 'App\Entity\Enum\UserStatus'

Prior to the support for enumerations, this was allowed by array constants:

App\Entity\User:
    properties:
        status:
            - Choice:
                choices: !php/const 'App\Entity\User::AVAILABLE_STATUSES'

[derrabus]

Please provide a meaningful PR description that explains the feature you're adding, how to use it and what's your motivation behind it. This helps us deciding whether we want to add your feature to Symfony. In addition to that, it helps the documentation team who will probably want to document your feature and announce it in our blog.

[phansys]

Please provide a meaningful PR description that explains the feature you're adding, how to use it and what's your motivation behind it. This helps us deciding whether we want to add your feature to Symfony. In addition to that, it helps the documentation team who will probably want to document your feature and announce it in our blog.

I've added an example with a use case. Please, let me know if more context is required.
Thank you.

[GromNaN]

Prior to the support for enumerations, this was allowed by array constants

It is still possible to add a constant to an enum.
The proposed syntax seems good and concise.

[fabpot]

Thank you @phansys.