[Java] Wrap Nullable Fields in Optional Class

**Is your feature request related to a problem? Please describe.**

If a field is nullable in the schema it's not reflected in the code generated by gql-gen.

```graphql
input NullableExample {
nullableField: String
notNullable: String!
}
```
```java
class MutationNameArgs {
private String _nullableField;
private String _notNullable;

public String getNullableField() { return this._nullableField; }
public String getNotNullable() { return this._notNullable; }
}
```

**Describe the solution you'd like**
It would be great if there was an option to have fields be `Optional`s if they are optional in the schema.

```java
class MutationNameArgs {
private String _nullableField;
private String _notNullable;

public Optional getNullableField() { return Optional.ofNullable(this._nullableField); }
public String getNotNullable() { return this._notNullable; }
}
```