<!-- The PR description should answer 2 important questions: -->

### What
```http
GET /v1/rest/default/Articles/?page[limit]=2&fields[Article]=title,author_id&fields[Author]=first_name&include=Author,Author.articles HTTP/1.1
```
<details>
<summary>Response</summary>

```json
{
  "data": [
    {
      "type": "default_Article",
      "id": "1",
      "attributes": {
        "author_id": 1,
        "title": "The Next 700 Programming Languages"
      },
      "relationships": {
        "Author": {
          "data": {
            "type": "default_Author",
            "id": "2"
          }
        }
      }
    },
    {
      "type": "default_Article",
      "id": "5",
      "attributes": {
        "author_id": 2,
        "title": "Why Functional Programming Matters"
      },
      "relationships": {
        "Author": {
          "data": {
            "type": "default_Author",
            "id": "6"
          }
        }
      }
    }
  ],
  "included": [
    {
      "type": "default_Article",
      "id": "3",
      "attributes": {
        "author_id": 1,
        "title": "The Next 700 Programming Languages"
      }
    },
    {
      "type": "default_Article",
      "id": "4",
      "attributes": {
        "author_id": 1,
        "title": "The Mechanical Evaluation of Expressions"
      }
    },
    {
      "type": "default_Author",
      "id": "2",
      "attributes": {
        "first_name": "Peter"
      },
      "relationships": {
        "articles": {
          "data": [
            {
              "type": "default_Article",
              "id": "3"
            },
            {
              "type": "default_Article",
              "id": "4"
            }
          ]
        }
      }
    },
    {
      "type": "default_Article",
      "id": "7",
      "attributes": {
        "author_id": 2,
        "title": "Why Functional Programming Matters"
      }
    },
    {
      "type": "default_Article",
      "id": "8",
      "attributes": {
        "author_id": 2,
        "title": "The Design And Implementation Of Programming Languages"
      }
    },
    {
      "type": "default_Article",
      "id": "9",
      "attributes": {
        "author_id": 2,
        "title": "Generalizing monads to arrows"
      }
    },
    {
      "type": "default_Author",
      "id": "6",
      "attributes": {
        "first_name": "John"
      },
      "relationships": {
        "articles": {
          "data": [
            {
              "type": "default_Article",
              "id": "7"
            },
            {
              "type": "default_Article",
              "id": "8"
            },
            {
              "type": "default_Article",
              "id": "9"
            }
          ]
        }
      }
    }
  ]
}
```
</details>

<!-- What is this PR trying to accomplish (and why, if it's not
obvious)? -->

<!-- Consider: do we need to add a changelog entry? -->

<!-- Does this PR introduce new validation that might break old builds?
-->

<!-- Consider: do we need to put new checks behind a flag? -->

### How
- Return the `include` parameter in the openapi schema
(`/v1/rest/__schema`).
- Resolve relationship fields in the OpenDD query request. Only local
relationships allowed for now.
- A parser to parse the `include` parameter with nested relationships
and build OpenDD query AST for relationship selection.
- Add tests
<!-- How is it trying to accomplish it (what are the implementation
steps)? -->

V3_GIT_ORIGIN_REV_ID: 516ca9f117f242892146d6026b35266cbaab591d