Add new Event serializer to return JSON only response based on FF

What does this MR do and why?

Creates a new serializer to return JSON response of Events based on the feature flag introduced in issue 37868

How to set up and validate locally

• Make an API request using Postman/CURL requesting JSON format: http://127.0.0.1:3000/users/root/activity?limit=5&format=json
• Expect a response similar to the following:

Sample response

[
  {
    "created_at": "2023-02-22T11:56:20.502Z",
    "action": "pushed",
    "ref": {
      "type": "branch",
      "count": null,
      "name": "new_branch",
      "path": "/le/ab/-/commits/new_branch"
    },
    "commit": {
      "truncated_sha": "ada0fe81",
      "path": "/le/ab/-/commit/ada0fe81494fb8fa59af54360cc6ba49c71446b9",
      "title": "Added a file",
      "count": 1,
      "create_mr_path": "/le/ab/-/merge_requests/new?merge_request%5Bsource_branch%5D=new_branch",
      "from_truncated_sha": null,
      "to_truncated_sha": "ada0fe81"
    },
    "author": {
      "id": 11,
      "name": "Gwyn Pollich",
      "path": "le"
    },
    "target": {
      "target_type": null,
      "title": null
    },
    "resource_parent": {
      "type": "project",
      "full_name": "Gwyn Pollich / __1",
      "full_path": "le/ab"
    }
  },
  {
    "created_at": "2023-02-22T11:56:20.501Z",
    "action": "pushed",
    "ref": {
      "type": "branch",
      "count": null,
      "name": "new_branch",
      "path": "/le/ab/-/commits/new_branch"
    },
    "commit": {
      "truncated_sha": "5b8ca6ce",
      "path": "/le/ab/-/commit/5b8ca6ce788c7140403ab78cba4ffff9166223b5",
      "title": "Added text",
      "count": 1,
      "create_mr_path": null,
      "from_truncated_sha": "ada0fe81",
      "to_truncated_sha": "5b8ca6ce"
    },
    "author": {
      "id": 11,
      "name": "Gwyn Pollich",
      "path": "le"
    },
    "target": {
      "target_type": null,
      "title": null
    },
    "resource_parent": {
      "type": "project",
      "full_name": "Gwyn Pollich / __1",
      "full_path": "le/ab"
    }
  },
  {
    "created_at": "2023-02-20T12:56:32.949Z",
    "action": "commented",
    "author": {
      "id": 11,
      "name": "Gwyn Pollich",
      "path": "le"
    },
    "target": {
      "target_type": "Note",
      "title": "1",
      "target_url": "/le/abc/-/issues/1#note_1399",
      "first_line_in_markdown": "<p>Hi</p>",
      "attachment": {
        "url": null
      }
    },
    "resource_parent": {
      "type": "project",
      "full_name": "Gwyn Pollich / .abc",
      "full_path": "le/abc"
    }
  },
  {
    "created_at": "2023-02-20T12:10:33.216Z",
    "action": "created",
    "author": {
      "id": 11,
      "name": "Gwyn Pollich",
      "path": "le"
    },
    "target": {
      "target_type": "Issue",
      "title": "1",
      "target_url": "/le/abc/-/issues/1",
      "reference_link_text": "#1"
    },
    "resource_parent": {
      "type": "project",
      "full_name": "Gwyn Pollich / .abc",
      "full_path": "le/abc"
    }
  },
  {
    "created_at": "2023-02-17T02:50:26.040Z",
    "action": "pushed",
    "ref": {
      "type": "branch",
      "count": null,
      "name": "main",
      "path": "/le/ab/-/commits/main"
    },
    "commit": {
      "truncated_sha": "f7cddfd1",
      "path": "/le/ab/-/commit/f7cddfd1c31f17879f15fd7c799a7e34fc5b97d3",
      "title": "Add new file",
      "count": 1,
      "create_mr_path": null,
      "from_truncated_sha": "21082f19",
      "to_truncated_sha": "f7cddfd1"
    },
    "author": {
      "id": 11,
      "name": "Gwyn Pollich",
      "path": "le"
    },
    "target": {
      "target_type": null,
      "title": null
    },
    "resource_parent": {
      "type": "project",
      "full_name": "Gwyn Pollich / __1",
      "full_path": "le/ab"
    }
  }
]

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #378695 (closed)