Fix object/map field stitching

[mattjohnsonpint]

Description

Fixes an issue where an output object with a nested output object wasn't being materialized correctly.

For example, a function returning a map of maps:

export function testMapOfMaps(): Map<string, Map<string, f64>> {
  const m = new Map<string, Map<string, f64>>();
  const inner = new Map<string, f64>();
  inner.set("a", 1);
  inner.set("b", 2);
  m.set("inner", inner);
  return m;
}

The error caused the internal input template to only provide the root map's key and value fields. The result of filling that template would then be a GraphQL error, such as:

{ "errors": [
        {
            "message": "Array cannot represent non-array value.",
            "path": [
                "testMapOfMaps",
                0,
                "value",
                "value"
            ]
        }
    ]
}

The root cause turned out to be a simple struct copy issue.

No tests, because we still need to make a test suite for the GraphQL datasource.

Checklist

• Code compiles correctly and linting passes locally
• Tests for new functionality and regression tests for bug fixes added
• Documentation added or updated
• Entry added to the CHANGELOG.md file describing and linking to this PR