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>'