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

This PR adds v3 of the AuthConfig, which will allow users to configure
the headers in the auth webhook.

We are also allowing users to configure the auth hook URL using env
vars.

- [x] Figure out what we should do for POST auth hooks
- [ ] Update docs

### What

This PR adds AuthConfig v3.

GET:
```json
{
  "version": "v3",
  "definition": {
    "mode": {
      "webhook": {
        "method": "GET",
        "url": {
          "value": "http://auth_hook:3050/validate-request"
        },
        "customHeadersConfig": {
          "headers": {
            "forward": ["Authorization"],
            "additional": {
              "user-agent": "hasura-ddn"
            }
          }
        }
      }
    }
  }
}
```

POST:
```json
{
  "version": "v3",
  "definition": {
    "mode": {
      "webhook": {
        "method": "POST",
        "url": {
          "value": "http://auth_hook:3050/validate-request"
        },
        "customHeadersConfig": {
          "headers": {
            "additional": {
              "user-agent": "hasura-ddn"
            }
          },
          "body": {
            "headers": {
              "forward": "*"
            }
          }
        }
      }
    }
  }
}
```

This config allows us to configure the headers sent to the auth
webhooks.

**Note**: Other auth mode configuration remains the same.

### How

<!-- How is it trying to accomplish it (what are the implementation
steps)? -->

---------

Co-authored-by: Daniel Chambers <daniel@hasura.io>
V3_GIT_ORIGIN_REV_ID: 4c86aeb3b0a335fe33bd24bb536fb3d98f501cdb