Feature request: AWS::Serverless::GraphQLApi EnvironmentVariables Property

### Describe your idea/feature/enhancement

Please add the `EnvironmentVariables` as a property on the `AWS::Serverless::GraphQLApi` resource type. Currently the only way to set this is to instead use the `AWS::AppSync::GraphQLApi` resource type.

[Serverless GraphQL Resource Type Documentation](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-resource-graphqlapi.html#sam-resource-graphqlapi-syntax)

[AppSync GraphQL Resource Type Documentation](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-appsync-graphqlapi.html#aws-resource-appsync-graphqlapi-syntax)

### Proposal

Pass through the property from the `::Serverless::` resource type -> `::AppSync::` resource type in the transform.

### Additional Details

This will allow us to add environment variables to our AppSync's JS resolvers.