Add a directive to hide a field from introspection

We really need to have a directive to choose the field I want to hide from introspection, something like:

```graphql
type Mutation {

# This mutation is shown in introspection query.
shown(input: Input): Output

# This mutation is hidden from introspection query.
hidden(input: Input): Output @aws_hide_from_introspection

}
```

This is somewhat related to #20

I'm using `@deprecated` as an ugly workaround:

```graphql
type Mutation {

# This mutation is shown in introspection query.
shown(input: Input): Output

# This mutation is hidden from introspection query.
hidden(input: Input): Output @deprecated(reason: "Hidden")

}
```

`@deprecated` makes the field "hard" to find in GraphQL explorer

Start by reviewing the requested directive behavior in this issue and the related issue #20. Confirm how field introspection currently works in AWS AppSync and determine the design and compatibility requirements before implementation; done means a supported directive hides only the selected field from introspection without affecting its use.

Written by the indexing model from the issue text.