feat: allow for editing the bot's banner

[AstreaTSS]

Pull Request Type

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

Description

This PR allows you to edit the bot's banner, like how you can for the icon right now.

Changes

• Add banner attribute to ClientUser.edit. This is pretty much a copy and paste of avatar.
  • The docstrings have been updated to note this.
• Update user.pyi.

Related Issues

Fixes #1635.

Test Scenarios

@slash_command()
async def test(ctx: SlashContext):
    await ctx.send("Hello!")
    await ctx.bot.user.edit(banner=...)

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