refactor(metadata): provide a trait in addition to the attribute

[soyuka]

Backport of https://github.com/api-platform/core/commit/9c461626f7d02f8d15487134b636f11966c19a5e

To add a few informations:

• this provides an IsApiResource trait that can be used in place of the #[ApiResource] attribute
• this also enables #[ApiProperty(property: 'foo')] on a class