Enhancement: Adds return types on ClassMetadata serialization methods

[mvhirsch]

Fixes:

Method "Metadata\ClassMetadata::serialize()" might add "string" as a native return type declaration in the future. Do the same in child class "Hateoas\Configuration\Metadata\ClassMetadata" now to avoid errors or add an explicit @return annotation to suppress this message

Method "Metadata\ClassMetadata::unserialize()" might add "void" as a native return type declaration in the future. Do the same in child class "Hateoas\Configuration\Metadata\ClassMetadata" now to avoid errors or add an explicit @return annotation to suppress this message.

[goetas]

can you please check phpcs failures?

[mvhirsch]

can you please check phpcs failures?

Fixed in #341

[mvhirsch]

@goetas Rebased 👍