Pass default argument and input values to resolvers

AWS AppSync allows to define [default values](https://graphql.github.io/graphql-spec/June2018/#sec-Input-Objects) in schema like that

```graphql
type Foo {
bar(
arg: Int = 20
): Bar!
}
```

or that

```graphql
type Foo {
bar(
arg: Int! = 20
): Bar!
}
```

But either way when the value is not explicitly defined, the default value is not passed to the resolver.

The default argument and input values defined in the schema should be passed to the resolvers.

As a workaround I'm forced to define default values at the schema level **and** at the resolver level. This leads to redundancy and to possible out of sync default values.

PS: In the [GraphQL specs](https://graphql.github.io/graphql-spec/June2018/#sec-Input-Objects)

> If no value is provided for a defined input object field and that field definition provides a default value, the default value should be used. If no default value is provided and the input object field’s type is non‐null, an error should be thrown. Otherwise, if the field is not required, then no entry is added to the coerced unordered map.

See Also https://stackoverflow.com/questions/51302462/how-to-pass-default-graphql-arguments-to-aws-appsync-resolver

The issue names AWS AppSync resolvers and GraphQL schema defaults, but no repository file, test, or implementation entry point. Start by determining whether resolver argument coercion is implemented in this repository or controlled by AppSync; done means omitted arguments and input fields reach resolvers with their schema-defined defaults, covering both nullable and non-null examples.

Written by the indexing model from the issue text.