Add update avatar on user api

What does this MR do and why?

Add Update avatar on user api

Endpoint: user/avatar

This MR allows users to customize avatars for group and project access token bots.

Connected to : Allow avatar customization for group or project... (#356868 - closed)

How to set up and validate locally

curl --request PUT \
  --url http://gdk.test:3000/api/v4/user/avatar \
  --header 'Authorization: Bearer <PROJECT_OR_GROUP_BOT_ACCESS_TOKEN' \
  --header 'Content-Type: multipart/form-data' \
  --form 'avatar=@<PATH_TO_FILE>'

doc/api/users.md

@@ -2543,3 +2543,43 @@ Example response:
     "token_expires_at": null
 }
 ```
+
+## Upload a current user avatar
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/148130) in GitLab 17.0.
+
+Upload an avatar to current user.
+
+```plaintext
+PUT /user/avatar
+```
+
+| Attribute | Type              | Required | Description                                                                                                 |
+|-----------|-------------------|----------|-------------------------------------------------------------------------------------------------------------|
+| `avatar`  | string            | Yes      | The file to be uploaded. The ideal image size is 192 x 192 pixels. The maximum file size allowed is 200 KiB. |
+
+To upload an avatar from your file system, use the `--form` argument. This causes
+cURL to post data using the header `Content-Type: multipart/form-data`. The
+`file=` parameter must point to an image file on your file system and be
+preceded by `@`. For example:
+
+Example request:
+
+```shell
+curl --request PUT --header "Bearer: <your_access_token>" \
+     --form "avatar=@avatar.png" \
+     --url "https://gitlab.example.com/api/v4/user/avatar"
+```
+
+Returned object:
+
+Returns `400 Bad Request` for file sizes greater than 200 KiB.
+
+If successful, returns [`200`](rest/index.md#status-codes) and the following
+response attributes:
+
+```json
+{
+  "avatar_url": "http://gdk.test:3000/uploads/-/system/user/avatar/76/avatar.png",
+}
+```