Decouple from api-platform/symfony

[Rainrider]

Having this bundle coupled to api-platform/symfony is inconvenient for a service not needing Api Platform. I.e.

composer req mtarld/api-platform-ms-bundle

fails when the package installing it has a dependency on doctrine, because of this. This must be then resolved by the user with

composer req mtarld/api-platform-ms-bundle api-platform/doctrine-orm

I'm not sure if I haven't overlooked something, but the only two services from Api Platform, which this bundle depends on, are api_platform.iri_converter and api_platform.validator. See #92 about removing those. After that, I believe api-platform/symfony is not a needed dependency any more. What do you think? Have I missed something?