Add session argument for GraphQLApp() constructor

[conao3]

• Add session argument for GraphQLApp() constructor to allow
  specify sqlalchemy session.

---

see graphene doc.

Now we use this method,

Create a query for the models.

Base = declarative_base()
Base.query = db_session.query_property()

class MyModel(Base):
    # ...

but below method cannot use.

Set the db session when you do the execution:

schema = graphene.Schema()
schema.execute(context_value={'session': session})

This patch allows to use latter method like this.

schema = graphene.Schema(query=Query)
app = starlette.graphql.GraphQLApp(schema=schema, session={{your SA session}})

[Kludex]

GraphQL's support in Starlette will be removed in the next release (0.17.0). Check: #1198