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

@@ -2556,7 +2556,7 @@ 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 200KiB. |
+| `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