fix: add avatar_url to user Mixins

[maksims8]

Pull Request Type

• Feature addition
• Bugfix
• Documentation update
• Code refactor
• Tests improvement
• CI/CD pipeline enhancement
• Other: [Replace with a description]

Description

Changes

Add avatar_url property to Mixins in user.pyi

Related Issues

Implements issue: #1525

Test Scenarios

import interactions

intents = interactions.Intents.DEFAULT
client = interactions.Client(intents=intents)

@interactions.listen()
async def on_ready():
    print("Client has connected to Discord.")

@interactions.slash_command("test", "A test command")
@interactions.slash_user_option("User to get avatar from.", required=True)
async def test_command(ctx: interactions.SlashContext, user: interactions.User):
    await ctx.defer()
    user_avatar = user.avatar_url
    await ctx.respond(user_avatar)

client.start("TOKEN")

Python Compatibility

• I've ensured my code works on Python 3.10.x
• I've ensured my code works on Python 3.11.x

Checklist

• I've run the pre-commit code linter over all edited files
• I've tested my changes on supported Python versions
• I've added tests for my code, if applicable
• I've updated / added documentation, where applicable