SchemaUtils Nested Map Fix

[jescalada]

This PR fixes the issue where files generated by 3rd-party tools (with nonstandard schemas) fail to read correctly.

Let me know if the new test is comprehensive enough. I tried to add another test by generating a nested schema recursively, but this would require modifying DoRoundtripTest by changing the keys/values from 2D to 3D arrays.

This was the schema generation function I thought of, but I don't know if this is overkill for our purposes.

        private static GroupNode CreateLogicalTypeMapSchema(bool nested)
        {
            using var stringType = LogicalType.String();
            using var mapType = LogicalType.Map();

            using var keyNode = new PrimitiveNode("key", Repetition.Required, stringType, PhysicalType.ByteArray);
            using var primitiveValueNode = new PrimitiveNode("value", Repetition.Optional, stringType, PhysicalType.ByteArray);
            using var valueNode = nested
                ? new GroupNode("value", Repetition.Optional, new Node[] {keyNode, primitiveValueNode}, mapType)
                : (Node)primitiveValueNode;
            using var keyValueNode = new GroupNode(
                "key_value", Repetition.Repeated, new Node[] {keyNode, valueNode}, mapType);
            using var colNode = new GroupNode(
                "col1", Repetition.Required, new Node[] {keyValueNode}, mapType);

            return new GroupNode(
                "schema", Repetition.Required, new Node[] {colNode});
        }

[adamreeve]

This looks good thanks Juan, I just have some comments about the test