GraphQL field ordering is not respected in REST endpoint OpenAPI schema

### Version Information

Server Version: v2.1.1

### Environment

OSS

### What is the expected behaviour?
The order of the properties in the OpenAPI schema should match the order they are defined in the GraphQL query.

### Keywords

Swagger, OpenAPI, GraphQL, REST Endpoint, Ordering

### What is the current behaviour?
The fields are presented out of order sometimes, but not always.

### How to reproduce the issue?
If I define a REST endpoint with the following GraphQL query:

```graphql
query MyQuery {
User {
first_name
last_name
handle
}
}
```

I get an OpenAPI/Swagger schema for that endpoint with properties in the incorrect order (`handle` is second, not third):

```json
"items": {
"type": "object",
"properties": {
"first_name": {
"title": "String",
"type": "string"
},
"handle": {
"title": "String",
"type": "string"
},
"last_name": {
"title": "String",
"type": "string"
}
}
},
```

### Can you identify the location in the source code where the problem exists?

I suspect the mis-ordering is happening at JSON encoding time, because the order of the properties inside the Schema is correct when it is created in `Hasura.Server.OpenAPI.declareOpenApiSpec`. However, a cursory look over the JSON encoding logic seems like it _ought_ to work correctly. There is something subtle going on here.