Allow different Scalar for GraphQLScalarValue

[jmpunkt]

Motivation

Currently it is not possible to use scalar values with custom scalars. The following code fails.

#[derive(juniper::GraphQLScalarValue)]
pub struct LargeId(i64);

In order to allow custom scalars, scalars values must allow to set a different scalar value. Currently the implementation only uses generics. The following snippet works with this PR. Notice that MyScalarValue is a scalar implementation including the i64 data type.

#[derive(juniper::GraphQLScalarValue)]
#[graphql(Scalar = MyScalarValue)]
pub struct LargeId(i64);

[LegNeato]

Looks great, thank you so much for the high quality PR with tests! 🍻