Make it possible for scalar annotation to register Doctrine and PHP types to map

[bartv2]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Documented?	not yet
License	MIT

When creating a scalar class it should be easy to also tell which types always map to this scalar. When the idea is accepted i will also update the documentation.

[Vincz]

Hey @bartv2 !

I like the PHP types <-> Scalar conversion, but I don't like the fact that we would need to dive into the app code to check which PHP type convert to which Scalar.
At the moment, we can add Doctrine types conversion with https://github.com/overblog/GraphQLBundle/blob/master/docs/annotations/index.md#field-type-auto-guessing-from-doctrine-orm-annotations.
Can't we do what you want in the config as well ?
Something like :

overblog_graphql:
    php:
        types_mapping:
            DateTime:  ScalarWithTypeMapping

Or maybe, am I missing something ?

[bartv2]

I was adding a scalar type and it wasn't working directly. These mappings are only used for the auto-guessing, i think. And with annotations (most of) the config will already be in the php code.

This PR also makes it possible to register the doctrine mapping in the annotation, maybe you didn't notice that. So all the mapping data can still be in one place.

Adding the php mapping to the config is a good idea for those who prefer that.