[Config] Allow enum values in EnumNode

[fancyweb]

Q	A
Branch?	6.3
Bug fix?	no
New feature?	yes
Deprecations?	no
Tickets	-
License	MIT
Doc PR	-

This PR allows adding 1..N case(s) from the same enum to the allowed values of EnumNode:

// Allow all cases
->enumNode('foo', FooEnum::cases())

// Allow some cases with some other scalar values
->enumNode('foo', [true, false, FooEnum::Foo, FooEnum::Bar])

The two benefits are not having to reconstruct the enum instance at process time and being able to use the real enum directly in PHP config files:

$config
    ->foo(FooEnum::Bar);

YAML config files can also pass an enum value:

foo: !php/enum \App\Enum\FooEnum::Bar

AFAIK, XML config files can't pass an enum value, which would mean they could not be used in this case.

[nicolas-grekas]

for xml, it's possible to create and reference an enum parameter I suppose?

[fabpot]

Thank you @fancyweb.